Optical pick-up head, optical information apparatus, and optical information reproducing method

ABSTRACT

An optical information apparatus of the present invention includes: an optical pick-up head including: a light source; a diffraction unit; a condensing unit; a beam splitter; a photodetector; and a tracking error signal generator. An optical recording medium has tracks arranged substantially at a constant pitch. An average of a pitch is tp. When a main beam is placed on the track, a first sub-beam and a second sub-beam are placed between the tracks. The tracking error signal generator performs a differential arithmetic operation with respect to signals output from a light-receiving portion receiving the main beam to generate a first push-pull signal, performs a differential arithmetic operation with respect to signals output from the light-receiving portions receiving the first sub-beam and the second sub-beam to generate a second push-pull signal, and performs a differential arithmetic operation with respect to the first push-pull signal and the second push-pull signal to generate a tracking error signal, in a case where an amplitude of the first push-pull signal obtained at the pitch tp is fluctuated when the light beam is scanned in a direction orthogonal to the tracks of the optical recording medium.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of application Ser. No. 10/646,602, filedAug. 22, 2003, which application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pick-up head for recording,reproducing or erasing information with respect to an optical recordingmedium on which information is recorded with a mark and a space, anoptical information apparatus, and an information reproducing method.

2. Description of the Related Art

Recently, as high-density and large-capacity recording media,high-density and large-capacity optical disks called DVDs have been putinto practical use, and spread widely as information media that can dealwith a large amount of information such as animation.

FIG. 70 shows a configuration of a general optical system used in anoptical pick-up head in an optical disk system as an optical informationapparatus capable of recording/reproducing information. According to aconventional configuration, an optical recording medium is irradiatedwith three light beams to detect a tracking error signal (e.g., seepages 5 to 8 and FIG. 2 of JP 3(1991)-005927 A).

A light source 1 such as a semiconductor laser emits a linearlypolarized divergent beam 70 having a wavelength λ1 of 405 nm. Thedivergent beam 70 emitted from the light source 1 is converted intoparallel light by a collimator lens 53 with a focal length f1 of 15 mm.Thereafter, the beam 70 is incident upon a polarized beam splitter 52.The beam 70 incident upon the polarized beam splitter 52 passes throughthe polarized beam splitter 52 and passes through a quarter-wavelengthplate 54 to be converted to a circularly polarized beam. Then, the beam70 is converted into a convergent beam by an objective lens 56 with afocal length f2 of 2 mm, passes through a transparent substrate 40 a ofan optical recording medium 40, and is condensed onto an informationrecording surface 40 b. An opening of the objective lens 56 is limitedby an aperture 55, and a numerical aperture NA is set to be 0.85. Thethickness of the transparent substrate 40 a is 0.1 mm. The opticalrecording medium 40 has an information recording surface 40 b. Theoptical recording medium 40 is provided with a continuous groove to be atrack, and a pitch tp is 0.32 μm.

The beam 70 reflected from the information recording surface 40 b passesthrough the objective lens 56 and the quarter-wavelength plate 54 to beconverted to linearly polarized light whose plane of polarization isshifted by 90° from an ingoing path. Then, the beam 70 is reflected fromthe polarized beam splitter 52. The beam 70 reflected from the polarizedbeam splitter 52 passes through a condensing lens 59 with a focal lengthf3 of 30 mm to be converted to convergent light. The resultant beam 70passes through a cylindrical lens 57 to be incident upon a photodetector30. The beam 70 is provided with astigmatism when passing through thecylindrical lens 57.

The photodetector 30 has four light-receiving portions 30 a to 30 d. Thelight-receiving portions 30 a to 30 d output current signals I30 a toI30 d in accordance with the respective light amounts received.

A focus error (hereinafter, referred to as a “FE”) signal according toan astigmatism method is obtained by (I30 a+I30 c)−(I30 b+I30 d). Atracking error (hereinafter, referred to as a “TE”) signal according toa push-pull method is obtained by (I30 a+I30 d)−(I30 b+I30 c).Information (hereinafter, referred to as a “RF”) signal recorded on theoptical recording medium 40 is obtained by I30 a+I30 b+I30 c+I30 d. TheFE signal and the TE signal are supplied to actuators 91 and 92 afterbeing amplified to a desired level and compensated for a phase, wherebyfocus and tracking control is performed.

When a pitch is reduced so as to increase the capacity of one opticalrecording medium 40 for recording information, the precision forproducing a track also must be enhanced accordingly. However, actually,an absolute amount of error is present, so that when a pitch isnarrowed, a production error amount with respect to a pitch isrelatively increased. Thus, compared with a DVD, the influence of thiserror is very large.

FIG. 71 shows a TE signal obtained by scanning the beam 70 in adirection orthogonal to tracks formed on the optical recording medium40. Tn−4, . . . , Tn+4 on a horizontal axis represent tracks formed onthe information recording surface 40 b of the optical recording medium40. In FIG. 71, solid lines extending in a vertical direction representcentral positions of the respective tracks Tn−4, . . . , Tn+4 in thecase where a pitch tp is formed uniformly. Herein, the track Tn−1 isformed at a position shifted by Δn−1 from a position where the trackTn−1 is supposed to be formed, and the track Tn is formed at a positionshifted by Δn from a position where the track Tn is supposed to beformed. Δn−1 is +25 nm, and Δn is −25 nm. As a result, the TE signalexhibits a maximum amplitude a and a minimum amplitude b in the vicinityof the track Tn−1. Thus, the TE signal fluctuates greatly. Furthermore,the position of a zero-intersection point of the TE signal is shifted byan off-track oft1 in the track Tn−1 and by an off-track oft2 in thetrack Tn from the centers of the tracks Tn−1 and Tn, respectively. Morespecifically, the off-tracks oft1 and oft2 represent off-track amounts.

Assuming that a fluctuation amount of the TE signal amplitude is definedas ΔPP=(amplitude a−amplitude b)/(amplitude a+amplitude b), and a TEsignal is detected by the above-mentioned conventional configuration,the fluctuation amount ΔPP is 0.69, the off-track oft1 is +33 nm, andthe off-track oft2 is −33 nm. Thus, the fluctuation amount and theoff-track are large. When the fluctuation amount ΔPP of the TE signalamplitude is large, the gain of tracking control is decreased in thetracks Tn−1 and Tn. As a result, tracking control becomes unstable, andinformation cannot be recorded/reproduced with high reliability.

SUMMARY OF THE INVENTION

Therefore, with the foregoing in mind, it is an object of the presentinvention to provide an optical pick-up head capable ofrecording/reproducing information with high reliability while reducingfluctuation in a TE signal amplitude, an optical information apparatus,and an information reproducing method.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a diffraction unit receiving a beam emitted from the light sourceto generate a plurality of diffracted beams composed of a 0th orderdiffracted light beam and a 1st or higher order diffracted light beam; acondensing unit receiving the plurality of diffracted beams from thediffraction unit and condensing the beams onto an optical recordingmedium; a beam splitter receiving the plurality of beams reflected fromthe optical recording medium and splitting the beams; and aphotodetector receiving the beams split by the beam splitter andoutputting signals in accordance with amounts of the received lightbeams. The 0th order diffracted light beam generated in the diffractionunit is set to be a main beam, and two 1st or higher order diffractedlight beams generated in the diffraction unit are set to be a firstsub-beam and a second sub-beam. The photodetector has a plurality oflight-receiving portions, and the main beam, the first sub-beam, and thesecond sub-beam are received by the plurality of light-receivingportions, respectively. The apparatus further includes a tracking errorsignal generator generating a tracking error signal for irradiating adesired track with a beam, wherein the optical recording medium hastracks arranged substantially at a constant pitch, and an average of thepitch is tp. When the main beam is placed on the track, the firstsub-beam and the second sub-beam are placed between the tracks, thetracking error signal generator performs a differential arithmeticoperation with respect to signals output from the light-receivingportion receiving the main beam to generate a first push-pull signal,performs a differential arithmetic operation with respect to signalsoutput from the light-receiving portions receiving the first sub-beamand the second sub-beam to generate a second push-pull signal, andperforms a differential arithmetic operation with respect to the firstpush-pull signal and the second push-pull signal to generate thetracking error signal, in a case where an amplitude of the firstpush-pull signal obtained at the pitch tp is fluctuated when the lightbeam is scanned in a direction orthogonal to the tracks of the opticalrecording medium.

The first push-pull signal is generated without using a region in avicinity of a center of the main beam, and the second push-pull signalis generated without using regions in a vicinity of centers of the firstsub-beam and the second sub-beam.

FThe above-mentioned optical information apparatus further includes aspherical aberration error signal generator generating a sphericalaberration error signal representing a spherical aberration amount of abeam condensed onto the optical recording medium. The sphericalaberration error signal generator performs a differential arithmeticoperation of the signals output from the plurality of light-receivingportions receiving a region in a vicinity of a center of the main beamto generate a first focus error signal, performs a differentialarithmetic operation of the signals output from the plurality oflight-receiving portions receiving a region in a vicinity of an outerside of the main beam to generate a second focus error signal, andperforms a differential arithmetic operation of the first focus errorsignal and the second focus error signal to obtain the sphericalaberration error signal.

The main beam, the first sub-beam, and the second sub-beam are receivedby four light-receiving portions, respectively, and the first push-pullsignal and the second push-pull signal are obtained by an arithmeticoperation (I1−I2)−k·(I3−I4) where I1 to I4 are outputs from the fourlight-receiving portions receiving the main beam, the first sub-beam,and the second sub-beam, respectively, and k is a real number.

The above-mentioned optical information apparatus further includes aspherical aberration error signal generator generating a sphericalaberration error signal representing a spherical aberration amount of abeam condensed onto the optical recording medium. The sphericalaberration error signal generator performs a differential arithmeticoperation of the signals output from the plurality of light-receivingportions receiving a region in a vicinity of a center of the main beamto generate a first focus error signal, performs a differentialarithmetic operation of the signals output from the plurality oflight-receiving portions receiving a region in a vicinity of an outerside of the main beam to generate a second focus error signal, andperforms a differential arithmetic operation of the first focus errorsignal and the second focus error signal to obtain the sphericalaberration error signal.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thelight received beams. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam. The optical recording mediumhas tracks arranged substantially at a constant pitch, an average of thepitch is tp, the beams are received by the plurality of light-receivingportions, and the tracking error signal generator performs adifferential arithmetic operation with respect to the signals outputfrom the light-receiving portions to generate a push-pull signal, and-in a case where an amplitude of the push-pull signal, obtained at apitch tp when the light beam is scanned in a direction orthogonal to thetracks of the optical recording medium, is changed at a pitch differentfrom the pitch tp, the push-pull signal is obtained by an arithmeticoperation (I1−I2)−k·(I3−I4) where I1 to I4 are the outputs from fourlight-receiving portions receiving the beams and k is a real number.

The above-mentioned optical information apparatus further includes aspherical aberration error signal generator generating a sphericalaberration error signal representing a spherical aberration amount of abeam condensed onto the optical recording medium. The optical recordingmedium has tracks arranged substantially at a constant pitch, an averageof the pitch is tp, the spherical aberration error signal generatorperforms a differential arithmetic operation of the signals output fromthe plurality of light-receiving portions receiving a region in avicinity of a center of the main beam to generate a first focus errorsignal, performs a differential arithmetic operation of the signalsoutput from the plurality of light-receiving portions receiving a regionin a vicinity of an outer side of the main beam to generate a secondfocus error signal, and performs a differential arithmetic operation ofthe first focus error signal and the second focus error signal to obtainthe spherical aberration error signal.

The tracking error signal generator generates the push-pull signalwithout using the region in the vicinity of the center of the beam.

The above-mentioned optical information apparatus further includes aspherical aberration error signal generator generating a sphericalaberration error signal representing a spherical aberration amount of abeam condensed onto the optical recording medium. The optical recordingmedium has tracks arranged substantially at a constant pitch, an averageof the pitch is tp. The spherical aberration error signal generatorperforms a differential arithmetic operation of the signals output fromthe plurality of light-receiving portions receiving a region in avicinity of a center of the main beam to generate a first focus errorsignal, performs a differential arithmetic operation of the signalsoutput from the plurality of light-receiving portions receiving a regionin a vicinity of an outer side of the main beam to generate a secondfocus error signal, and performs a differential arithmetic operation ofthe first focus error signal and the second focus error signal to obtainthe spherical aberration error signal.

Light passing through a region containing a large amount of 1st orderdiffracted light diffracted by the tracks of the optical recordingmedium is received by the light-receiving portions, whereby the outputsI1 and I2 are output from the light-receiving portions. Light passingthrough a region containing almost no 1st order diffracted lightdiffracted by the tracks of the optical recording medium is received bythe light-receiving portions, the outputs I3 and I4 are output from thelight-receiving portions, and in an image of the beam condensed on thecondensing unit, assuming that a distance of the region in the vicinityof the center of the beam that is not used for generating the push-pullsignal from a center of the condensing unit is L1 and a minimum distanceof the 1st order diffracted light diffracted by the tracks of theoptical recording medium from the center of the condensing unit is L2,light having passed through a region between the distance L1 and thedistance L2 is received by the light-receiving portions, whereby theoutputs I3 and I4 are output from the light-receiving portions.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam. The optical recording mediumhas tracks arranged substantially at a constant pitch, an average of thepitch is tp, and the beams are received by the plurality oflight-receiving portions. The tracking error-signal generator performs adifferential-arithmetic operation with respect to the signals outputfrom the light-receiving portions to generate a push-pull signal, and ina case where an amplitude of the push-pull signal, obtained at a pitchtp when the light beam is scanned in a direction orthogonal to thetracks of the optical recording medium, is changed at a pitch differentfrom the pitch tp, the push-pull signal is generated without using aregion in a vicinity of a center of the beam. The optical informationapparatus further includes a spherical aberration error signal generatorgenerating a spherical aberration error signal representing a sphericalaberration amount of a beam condensed on the optical recording medium,the spherical aberration error signal generator performs a differentialarithmetic operation of the signals output from the plurality oflight-receiving portions receiving a region in a vicinity of a center ofthe main beam to generate a first focus error signal, performs adifferential arithmetic operation of the signals output from theplurality of light-receiving portions receiving a region in a vicinityof an outer side of the main beam to generate a second focus errorsignal, and performs a differential arithmetic operation of the firstfocus error signal and the second focus error signal to obtain thespherical aberration error signal.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam. The optical recording mediumhas tracks arranged substantially at a constant pitch, and an average ofthe pitch is tp, the beams are received by the plurality oflight-receiving portions. The tracking error signal generator performs adifferential arithmetic operation with respect to the signals outputfrom the light-receiving portions to generate a push-pull signal, and ina case where an amplitude of the push-pull signal, obtained at a pitchtp when the light beam is scanned in a direction orthogonal to thetracks of the optical recording medium, is changed at a pitch differentfrom the pitch tp, the push-pull signal is generated without using aregion in a vicinity of a center of the beam.

In an image of the beam condensed on the condensing unit, the region inthe vicinity of the center of the beam that is not used for generatingthe push-pull signal may form a shape symmetrical with respect to asegment parallel to the image of the tracks in the condensing unit.

In the image of the beam condensed on the condensing unit, the region inthe vicinity of the center of the beam that is not used for generatingthe push-pull signal may have a rectangular shape.

In the image of the beam condensed on the condensing unit, the region inthe vicinity of the center of the beam that is not used for generatingthe push-pull signal may have a square shape.

In the image of the beam condensed on the condensing unit, the region inthe vicinity of the center of the beam that is not used for generatingthe push-pull signal may have a spiral shape.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam; a focus error signal generatorgenerating a focus error signal for irradiating a desired focus positionwith a beam; a recording/non-recording detector detecting whether or notinformation is recorded at a position of the beam condensed on theoptical recording medium; and an amplitude controller controlling anamplitude of the tracking error signal with a coefficient k. Theamplitude controller is controlled by using a signal generated from therecording/non-recording detector and a signal generated from the focuserror signal generator.

The recording/non-recording detector detects an amplitude of a signalvaried depending upon a mark and a space recorded on the opticalrecording medium and a signal with a low frequency component obtained bya low-pass filter from a signal output from the photodetector, therebydetecting whether or not information is recorded at a position of thebeam condensed on the optical recording medium.

k may be set so that a change amount of the amplitude is minimum in acase where the amplitude of the push-pull signal, obtained at the pitchtp when the light beam is scanned in a direction orthogonal to thetracks of the optical recording medium, is changed at a pitch differentfrom the pitch tp,

k may be set so that a position of a light beam, where the push-pullsignal obtained at the pitch tp when the light beam is scanned in adirection orthogonal to the tracks of the optical recording medium is asubstantially zero-intersection point, is close to a center of thetrack.

Assuming that k1 is a value of k for minimizing a change amount in acase where the amplitude of the push-pull signal, obtained at the pitchtp when the light beam is scanned in a direction orthogonal to thetracks of the optical recording medium, is changed at a pitch differentfrom the pitch tp and k2 is a value of k in a case where a position of alight beam, where the push-pull signal obtained at the pitch tp when thelight beam is scanned in a direction orthogonal to the tracks of theoptical recording medium is a substantially zero-intersection point, isclosest to a center of the track, k may be set to be a value between k1and k2.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam. The optical recording mediumhas an information recording surface for recording information, theoptical recording medium has a reflective surface for reflecting thebeam when the beam is condensed onto the information recording surface,and the beam splitter has a plurality of regions. A size of the beam onthe beam splitter is D, a numerical aperture of the condensing unit isNA, a lateral multiplication of an optical system in the optical pick-uphead from the optical recording medium to the photodetector is α, aninterval between the information recording surface and the reflectivesurface is d, and a refractive index present in the interval d betweenthe information recording surface and the reflective surface is n2. Thetracking error signal generator performs a differential arithmeticoperation with respect to the signals output from the light-receivingportions to generate a push-pull signal, and when the beam splittersplits the beam in a direction different from that of thelight-receiving portion outputting a signal for generating the trackingerror signal over a width h of a region in a vicinity of a center to beirradiated with the beam, a width S of the light-receiving portionoutputting a signal for generating the tracking error signal has arelationship S≦2·h·α·NA·d/(D·n2).

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam. The optical recording mediumhas an information recording surface for recording information, and theoptical recording medium has a reflective surface for reflecting thebeam when the beam is condensed onto the information recording surface.The tracking error signal generator performs a differential arithmeticoperation with respect to the signals output from the light-receivingportions to generate a push-pull signal, and the beam splitter has fiveregions, and splits the beam in a direction different from that of thelight-receiving portion outputting a signal for generating the trackingerror signal over a width h of a region in a vicinity of a center to beirradiated with the beam and splits the beam in the substantially samedirection in the other four regions.

The condensing unit is driven in accordance with tracking control, thebeam splitter splits the beam in a direction substantially orthogonal toa direction in which an image on the light-receiving portion is movedwhen the condensing unit is driven, and the tracking error signal isgenerated with the split beams.

The beams split from the plurality of regions in the beam splitter arereceived by the plurality of light-receiving portions substantiallyadjacent to each other.

The beams split by first and second regions of the beam splitter containa large amount of 1st order diffracted light diffracted by the tracks ofthe optical recording medium, the beams split by third and fourthregions of the beam splitter contain almost no 1st order diffractedlight diffracted by the tracks of the optical recording-medium, and afirst virtual segment on the photodetector connecting the beam split bythe first region to the beam split by the second region, and a secondvirtual segment on the photodetector connecting the beam split by thethird region to the beam split by the fourth region is orthogonal to animage of the tracks on the photodetector, respectively.

An outline of the plurality of light-receiving portions substantiallyadjacent to each other may be a rectangle.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam. The optical recording mediumhas an information recording surface for recording information, and theoptical recording medium has a reflective surface for reflecting thebeam when the beam is condensed onto the information recording surface.The tracking error signal generator performs a differential arithmeticoperation with respect to the signals output from the light-receivingportions to generate a push-pull signal. The beam splitter has fivedifferent regions, and splits the beam in a direction different fromthat of the light-receiving portion outputting a signal for generatingthe tracking error signal over a width h of a region in a vicinity of acenter to be irradiated with the beam and splits the beam in thesubstantially same direction in the other four regions. Thephotodetector has five light-receiving portions at positions close toeach other, each of the beams split by the other four regions of thebeam splitter is received one light-receiving portion, and the trackingerror signal generator obtains the push-pull signal by an arithmeticoperation {(I1−I5)−k1·(I−I5)}−k·{(I3−I5)−k2·(I4−I5)}, where I1 to I4 aresignals output from the four light-receiving portions receiving thebeams split by the other four regions of the beam splitter, 15 is asignal output from the light-receiving portion provided close to thefour light-receiving portions receiving the beams split by the beamsplitter, and k is a real number.

The beams split by the beam splitter are substantially focused on thelight-receiving portion.

The light-receiving portion outputting a signal for detecting the focuserror signal is integrated with the light-receiving portion outputting asignal for detecting the tracking error signal.

The optical pick-up head further includes an astigmatism generator thatprovides a beam with astigmatism in an optical path from the opticalrecording medium to the photodetector, and the focus error signal isdetected based on the beam provided with the astigmatism.

The beam splitter provides a beam to be split with a wave front forcanceling the astigmatism provided to the beam by the astigmatismgenerator.

The region in the vicinity of the center of the beam that is not usedfor generating the push-pull signal corresponds to a region where the0th order diffracted light and the first order diffracted light, whichare reflected and diffracted by the optical recording medium, are notoverlapped with each other.

The plurality of light-receiving portions respectively receive a beampartially, thereby splitting the beam.

The beam is split by providing the beam splitter in an optical path fromthe optical recording medium to the photodetector.

The amplitude of the push-pull signal obtained at the pitch tp when thelight beam is scanned in a direction orthogonal to the tracks of theoptical recording medium is changed at a pitch different from the pitchtp, in a region where a track in which information is not recorded isadjacent to a track in which information is recorded.

The amplitude of the push-pull signal obtained at the pitch tp when thelight beam is scanned in a direction orthogonal to the tracks of theoptical recording medium is changed at a pitch different from the pitchtp, by fluctuation in a pitch formed on the optical recording medium.

The amplitude of the push-pull signal obtained at the pitch tp when thelight beam is scanned in a direction orthogonal to the tracks of theoptical recording medium is changed at a pitch different from the pitchtp, by fluctuation in a track width formed on the optical recordingmedium.

The amplitude of the push-pull signal obtained at the pitch tp when thelight beam is scanned in a direction orthogonal to the tracks of theoptical recording medium is changed at a pitch different from the pitchtp, by fluctuation in a track depth formed on the optical recordingmedium.

Assuming that tracks irradiated with the main beam when the main beam isscanned in a direction orthogonal to the tracks are Tn−1, Tn, and Tn+1,and when the main beam is placed at a center of the track Tn, the firstsub-beam is placed between the tracks Tn−1 and Tn, and the secondsub-beam is placed between the tracks Tn and Tn+1.

Assuming that tracks irradiated with the main beam when the main beam isscanned in a direction orthogonal to the tracks are Tn−2, Tn−1, Tn,Tn+1, and Tn+2, and when the main beam is placed at a center of thetrack Tn, the first sub-beam is placed between the tracks Tn−2 and Tn−1,and the second sub-beam is placed between the tracks Tn+1 and Tn+2.

A relationship tp/0.8<λ/NA<0.5 μm may be satisfied, where λ is awavelength of the light source, and NA is a numerical aperture of thecondensing unit.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam. The optical recording mediumhas an information recording surface for recording information, theoptical recording medium has a reflective surface for reflecting thebeam when the beam is condensed onto the information recording surface,and the light-receiving portions are placed so that the beam reflectedfrom the reflective surface when the beam is condensed onto theinformation recording surface is not incident upon the light-receivingportions.

The reflective surface for reflecting the beam when the beam iscondensed onto the information recording surface may be a secondinformation recording surface.

The reflective surface for reflecting the beam when the beam iscondensed onto the information recording surface may be a beam incidentsurface of the optical recording medium.

The light-receiving portion receiving a beam used for generating thetracking error signal may be smaller than the light-receiving portionsreceiving the other beams.

The optical recording medium has a plurality of information recordingsurfaces.

An optical information apparatus of the present invention includes: anoptical pick-up head that includes: a light source emitting a lightbeam; a spherical aberration providing unit providing the beam withspherical aberration; a condensing unit receiving the beam from thespherical aberration providing unit and condensing the beam onto anoptical recording medium; a beam splitter receiving the beam reflectedfrom the optical recording medium and splitting the beam; aphotodetector receiving the beams split by the beam splitter andoutputting signals in accordance with mounts of the received lightbeams; and a driving unit driving the condensing unit to enable trackingcontrol to be performed. The photodetector has a plurality oflight-receiving portions. The apparatus further includes a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam and an offset compensating unitcompensating an offset occurring in the tracking error signal inaccordance with a position of the condensing unit driven by the drivingunit. The spherical aberration providing unit is capable of adjustingspherical aberration provided to the beam in accordance with a state ofthe beam condensed on the optical recording medium, and the offsetcompensating unit is controlled in accordance with the sphericalaberration provided by the spherical aberration providing unit.

An optical information apparatus of the present invention includes: alight source emitting a light beam; a condensing unit condensing thelight beam emitted from the light source onto an optical recordingmedium having a track; a splitter splitting the light beamreflected/diffracted from the optical recording medium; a dividerdividing the split light beams into a plurality of regions; aphotodetector having a plurality of detection regions detecting lightbeams divided by the divider and outputting current signals inaccordance with amounts of the detected light beams; a plurality ofconverters converting the current signal output from the photodetectorto voltage signals; and a tracking error signal generator generating atracking error signal by subtracting a voltage signal obtained from asecond region multiplied by a coefficient from a voltage signal obtainedfrom a first region. Among the plurality of regions placed in thedivider, a region mainly containing a tracking error signal component isset to be the first region, and a region mainly containing an offsetcomponent of a tracking error signal is set to be the second region. Anefficiency at which a light beam having passed through the second regionreaches the photodetector is higher than an efficiency at which a lightbeam having passed through the first region reaches the photodetector.

An optical information apparatus of the present invention includes: alight source emitting a light beam; a condensing unit condensing thelight beam emitted from the light source onto an optical recordingmedium having a track; a splitter splitting the light beamreflected/diffracted from the optical recording medium; a dividerdividing the split light beams into a plurality of regions; aphotodetector having a plurality of detection regions detecting lightbeams split by the splitter and outputting current signals in accordancewith light amounts of the detected light beams; a plurality ofconverters converting the current signal output from the photodetectorto voltage signals; and a tracking error signal generator generating atracking error signal by converting a current signal obtained from alight beam having passed through the first region and a current signalobtained from a light beam having passed through the second region tovoltages by an identical converter. Among the plurality of regionsplaced in the divider, a region mainly containing a tracking errorsignal component is set to be the first region, and a region mainlycontaining an offset component of a tracking error signal is set to bethe second region.

An efficiency at which a light beam having passed through the secondregion reaches the photodetector is higher than an efficiency at which alight beam having passed through the first region reaches thephotodetector.

An efficiency at which a part of a light beam having passed through thesecond region reaches the photodetector is higher than an efficiency atwhich a light beam having passed through the first region reaches thephotodetector.

An efficiency at which a light beam having passed through an outercircumferential portion of the second region reaches the photodetectoris high.

An efficient at which a light beam having passed through acircumferential portion in a track tangent direction of the secondregion reaches the photodetector is high.

An efficiency at which a light beam having passed through acircumferential portion in a direction traversing a track in the secondregion reaches the photodetector is high.

The photodetector may include at least first to fourth detectionregions. The first region is divided into at least four regions by adividing line substantially parallel to a track tangent direction and adividing line substantially parallel to a direction orthogonal to thetracks. The second region is divided into at least four regions by adividing line substantially parallel to a track tangent direction and adividing line substantially parallel to a direction orthogonal to thetracks. Light having passed through the second region is converted to avoltage signal by the converter converting a current signal obtained byreceiving light having passed through regions in a diagonal direction ofthe first region.

The light having passed through the second region is condensed so as tobe focused on the photodetector.

The light having passed through the first region is condensed so as tobe focused on the photodetector.

A focus error signal and an information reproducing signal are generatedbased on the signals obtained by the photodetector having at least thefirst to fourth detection regions.

An optical information apparatus of the present invention includes: anoptical pick-up head including: a light source emitting a light beam; acondensing unit receiving a light beam from the light source andcondensing the light beam onto an optical recording medium; a beamsplitter splitting the light beam reflected/diffracted from the opticalrecording medium; a divider dividing the light beams, split by the beamsplitter, into a plurality of regions; a photodetector receiving thelight beams divided by the divider and outputting signals in accordancewith amounts of the received light beams; a tracking error signalgenerator generating a tracking error signal for irradiating a desiredtrack with the light beam; and an information signal generatorgenerating an information signal recorded on the optical recordingmedium. The tracking error signal generator performs a differentialarithmetic operation with respect to the signals output from thelight-receiving portions to generate a push-pull signal. The dividerdivides the light beam so as to generate the information signal and thepush-pull signal. The push-pull signal is generated based on signalsfrom regions other than a region in a vicinity of a center of the lightbeam, and a ratio of signals obtained from the region in the vicinity ofthe center of the light beam so as to generate the information signal ishigher than a ratio of signals obtained from a region on an outercircumference side of the light beam.

The region in the vicinity of the center of the light beam among theplurality of regions of the divider may have a rectangular shape.

The divider may be integrated with the condensing unit.

The divider may be a diffraction element, with a difference in thereaching efficiency is caused based on a difference in a diffractionefficiency of the diffraction element.

An optical pick-up head includes: a light source emitting a light beam;a diffraction unit receiving the light beam emitted from the lightsource to generate a plurality of diffracted beams composed of a 0thorder diffracted light beam and a 1st or higher order diffracted lightbeam; a condensing unit receiving the plurality of diffracted beams fromthe diffraction unit and condensing the beams onto an optical recordingmedium; a beam splitter receiving the plurality of beams reflected fromthe optical recording medium and splitting the beams; and aphotodetector receiving the beams split by the beam splitter andoutputting signals in accordance with amounts of the received lightbeams. The 0th order diffracted light beam generated in the diffractionunit is set to be a main beam, and two 1st or higher order diffractedlight beams generated in the diffraction unit are set to be a firstsub-beam and a second sub-beam, assuming that Tn−2, Tn−1 Tn, Tn+1, andTn+2 represent tracks irradiated with the main beam when the main beamis scanned in a direction orthogonal to the tracks, and the main beam isplaced at a center of the track Tn, the first sub-beam is placed betweenthe tracks Tn−2 and Tn−1, and the second sub-beam is placed between thetracks Tn+1 and Tn+2.

An optical pick-up head of the present invention includes: a lightsource emitting a light beam; a first condensing unit receiving thelight beam from the light source and condensing the light beam onto arecording surface of an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams; a beam divider dividing the beams split by thebeam splitter into a plurality of beams so as to correspond to aplurality of light-receiving portions placed on the photodetector; and asecond condensing unit condensing the beam onto the photodetector. Theoptical recording medium has a reflective surface for reflecting thebeam when the beam is condensed to the recording surface. An openinglimit member is provided between the first condensing unit and thesecond condensing unit, and an outer circumference portion of the beamreflected from the reflective surface for reflecting the beam of theoptical information recording medium is blocked against light so thatthe beam reflected from the reflective surface for reflecting the beamof the optical information recording medium is not incident upon thephotodetector.

The reflective surface for reflecting the beam when the beam iscondensed onto the recording surface may be formed on a beam incidentside with respect to the recording surface.

The reflective surface for reflecting the beam when the beam iscondensed onto the recording surface may be another recording surface ofthe optical recording medium.

The reflective surface for reflecting the beam when the beam iscondensed onto the recording surface may be a surface of the opticalrecording medium.

The opening limit member may be provided in a vicinity of the beamdivider.

The opening limit member may be integrated with the beam divider.

Even in a case where the condensing unit is displaced in a trackingdirection of the optical recording medium, an opening of the openinglimit member is larger in the tracking direction than in a directionorthogonal to the tracking direction, so as not to block light reflectedfrom the recording surface of the optical recording medium.

An optical information apparatus of the present invention includes: theabove-mentioned optical pick-up head; a driving portion changing arelative position between the optical recording medium and the opticalpick-up head; and an electric signal processing portion receivingsignals output from the optical pick-up head and performing adifferential arithmetic operation to obtain desired information.

A method for reproducing optical information of the present inventionuses an optical pick-up head that includes: a light source emitting alight beam; a condensing unit receiving a beam from the light source andcondensing the beam onto an optical recording medium; a beam splitterreceiving the beam reflected from the optical recording medium andsplitting the beam; and a photodetector receiving the beams split by thebeam splitter and outputting signals in accordance with amounts of thereceived light beams. The photodetector has a plurality oflight-receiving portions, and the method uses a tracking error signalgenerator generating a tracking error signal for irradiating a desiredtrack with a beam. The tracking error signal generator performs adifferential arithmetic operation with respect to the signals outputfrom the light-receiving portions to generate a push-pull signal. Theoptical recording medium has tracks arranged substantially at a constantpitch, and an average of the pitch is tp. When an amplitude of thepush-pull signal obtained at the pitch tp when the light beam is scannedin a direction orthogonal to the tracks of the optical recording mediumis changed: at a pitch different from the pitch tp, the change in theamplitude of the push-pull signal is reduced by avoiding use of apartial region of the beam or manipulating a signal obtained from thepartial region of the beam.

A method for reproducing optical information of the present inventionuses an optical pick-up head that includes: a light source emitting alight beam; a diffraction unit receiving the light beam emitted from thelight source to generate a plurality of diffracted beams composed of a0th order diffracted light beam and 1st or higher order diffracted lightbeams; a condensing unit receiving the plurality of beams from thediffraction unit and condensing the beams onto an optical recordingmedium; a beam splitter receiving the plurality of beams reflected fromthe optical recording medium and splitting the beams; and aphotodetector receiving the beams split by the beam splitter andoutputting signals in accordance with amounts of the received lightbeams. The method uses a tracking error signal generator generating atracking error signal for irradiating a desired track with a beam. Thephotodetector has a plurality of light-receiving portions, the pluralityof beams are radiated to positions different in a direction orthogonalto the tracks, the tracking error signal generator performs adifferential arithmetic operation with respect to the signals outputfrom the light-receiving portions to generate a push-pull signal, theoptical recording medium has tracks arranged substantially at a constantpitch, and an average of the pitch is tp. When an amplitude of thepush-pull signal obtained at the pitch tp when the light beam is scannedin a direction orthogonal to the tracks of the optical recording mediumis changed at a pitch different from the pitch tp, the change in theamplitude of the push-pull signal is reduced by manipulating a signalobtained from the plurality of beams.

A track in which information is not recorded and a track in whichinformation has been recorded may be formed previously on the opticalrecording medium so that the amplitude of the push-pull signal obtainedat the pitch tp when the light beam is scanned in a direction orthogonalto the tracks of the optical recording medium is changed at a pitchdifferent from the pitch tp.

The track in which information has been recorded and the track in whichinformation is not recorded may be placed alternately.

A method for reproducing optical information of the present inventionuses: a light source emitting a light beam; a condensing unit condensingthe light beam emitted from the light source onto an optical recordingmedium having a track; a splitter splitting the light beamreflected/diffracted from the optical recording medium; a dividerdividing the split light beams into a plurality of regions; aphotodetector having a plurality of detection regions detecting lightbeams divided by the divider and outputting current signals inaccordance with amounts of the detected light beams; a plurality ofconverters converting the current signal output from the photodetectorto voltage signals; and a tracking error signal generator generating atracking error signal by subtracting a voltage signal obtained from asecond region multiplied by a coefficient from a voltage signal obtainedfrom a first region. Among the plurality of regions placed in thedivider, a region mainly containing a tracking error signal component isset to be the first region, and a region mainly containing an offsetcomponent of a tracking error signal is set to be the second region. Anefficiency at which a light beam having passed through the second regionreaches the photodetector is higher than an efficiency at which a lightbeam having passed through the first region reaches the photodetector,whereby the offset component of the tracking error signal is reduced.

A method for reproducing optical information of the present inventionreduces an offset of a tracking error signal by including: a lightsource emitting a light beam; a condensing unit condensing the lightbeam emitted from the light source onto an optical recording mediumhaving a track; a splitter splitting the light beam reflected/diffractedfrom the optical recording medium; a divider dividing the split lightbeams into a plurality of regions; a photodetector having a plurality ofdetection regions detecting light beams divided by the divider andoutputting current signals in accordance with amounts of the detectedlight beams; a plurality of converters converting the current signaloutput from the photodetector to voltage signals; and a tracking errorsignal generator generating a tracking error signal by converting acurrent signal obtained from a light beam of a first region and acurrent signal obtained from a light beam of the second region tovoltages by an identical converter. Among the plurality of regionsplaced in the divider, a region mainly containing a tracking errorsignal component is set to be the first region, and a region mainlycontaining an offset component of a tracking error signal is set to bethe second region,

A method for reproducing optical information of the present inventionuses: an optical pick-up head that includes: a light source emitting alight beam; a condensing unit receiving a light beam from the lightsource and condensing the light beam onto an optical recording medium; abeam splitter splitting the light beam reflected/diffracted from theoptical recording medium; a divider dividing the light beams, split bythe beam splitter, into a plurality of regions; a photodetectorreceiving the beams divided by the divider and outputting signals inaccordance with light amounts of the received light beams; a trackingerror signal generator generating a tracking error signal forirradiating a desired track with the light beam; and an informationsignal generator generating an information signal recorded on theoptical recording medium. The tracking error signal generator performs adifferential arithmetic operation with respect to the signals outputfrom the light-receiving portions to generate a push-pull signal. Thedivider divides the light beam so as to generate the information signaland the push-pull signal. The push-pull signal is generated based onsignals from regions other than a region in a vicinity of a center ofthe light beam, and a ratio of information signals generated based onsignals from the region in the vicinity of the light beam is set to behigher than a ratio of information signals generated based on signalsfrom a region on an outer circumference side of the light beam, wherebyinformation recorded on the optical recording medium is reproduced.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a configuration of an optical informationapparatus of Embodiment 1 according to the present invention.

FIG. 2 shows a configuration of an optical pick-up head constituting theoptical information apparatus of Embodiment 1 according to the presentinvention.

FIG. 3 shows a relationship between a track and a beam on an opticalrecording medium in the optical information apparatus of Embodiment 1according to the present invention.

FIG. 4 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 1 according to the present invention.

FIG. 5 shows a state of a TE signal obtained in the optical informationapparatus of Embodiment 1 according to the present invention.

FIG. 6 shows a relationship between a track and a beam on an opticalrecording medium in an optical information apparatus of Embodiment 2according to the present invention.

FIG. 7 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 3 according to the present invention.

FIG. 8 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 4 according to the present invention.

FIG. 9 shows a configuration of an optical pick-up head in an opticalinformation apparatus of Embodiment 5 according to the presentinvention.

FIG. 10 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 5 according to the presentinvention.

FIG. 11 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 5 according to the present invention.

FIG. 12 shows a configuration of an optical pick-up head in an opticalinformation apparatus of Embodiment 6 according to the presentinvention.

FIG. 13 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 6 according to the presentinvention.

FIG. 14 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 6 according to the present invention.

FIG. 15 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in an optical information apparatusof Embodiment 7 according to the present invention.

FIG. 16 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 8 according to the presentinvention.

FIG. 17 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 8 according to the present invention.

FIG. 18 shows a configuration of a beam splitter constituting an opticalinformation apparatus of Embodiment 9 according to the presentinvention.

FIG. 19 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 9 according to the present invention.

FIG. 20 shows a configuration of an optical pick-up head constituting anoptical information apparatus of Embodiment 10 according to the presentinvention.

FIG. 21 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 10 according to the presentinvention.

FIG. 22 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 10 according to the present invention.

FIG. 23 shows a configuration of a signal processing part constitutingthe optical information apparatus of Embodiment 10 according to thepresent invention.

FIG. 24 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 11 according to the present invention.

FIG. 25 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 12 according to the present invention.

FIG. 26 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 13 according to the presentinvention.

FIG. 27 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 14 according to the presentinvention.

FIG. 28 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 15 according to the presentinvention.

FIG. 29 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 16 according to the presentinvention.

FIG. 30 shows a configuration of a signal processing part constitutingan optical information apparatus of Embodiment 17 according to thepresent invention.

FIG. 31 shows an information signal read from the optical informationapparatus of Embodiment 17 according to the present invention.

FIG. 32 shows a gain of a variable gain amplifier in a signal processingpart constituting the optical information apparatus of Embodiment 17according to the present invention.

FIG. 33 shows a relationship between a recorded track and a non-recordedtrack on an optical recording medium in the optical informationapparatus of Embodiment 17 according to the present invention.

FIG. 34 shows a configuration of an optical pick-up head constituting anoptical information apparatus of Embodiment 18 according to the presentinvention.

FIG. 35 shows a gain of a variable gain amplifier in a signal processingpart constituting the optical information apparatus of Embodiment 18according to the present invention.

FIG. 36 shows a relationship between a beam diameter and a drivingvoltage of an actuator in an optical pick-up head constituting theoptical information apparatus of Embodiment 18 according to the presentinvention.

FIG. 37 shows a configuration of an optical head apparatus in Embodiment19 according to the present invention.

FIG. 38 schematically shows an exemplary configuration of a diffractionelement and an opening limit element in Embodiment 19 according to thepresent invention.

FIG. 39 shows a relationship between an incident beam and the shape of alight-receiving surface of a photodetector in Embodiment 19 according tothe present invention.

FIG. 40 schematically shows another exemplary configuration of thediffraction element and the opening limit element in Embodiment 19according to the present invention.

FIG. 41 illustrates a configuration of an optical system of an opticalinformation apparatus of Embodiment 20 according to the presentinvention.

FIG. 42 shows a relationship between a light beam and division of anexemplary hologram element of the optical information apparatus ofEmbodiment 20 according to the present invention.

FIG. 43 shows a state of a change in a diffraction efficiency in atangential direction of the hologram element of the optical informationapparatus of Embodiment 20 according to the present invention.

FIG. 44 shows a relationship between a light beam and division of aphotodetector of the optical information apparatus of Embodiment 20according to the present invention, and a configuration of an electriccircuit.

FIG. 45 shows a relationship between a light beam and division ofanother exemplary hologram element of the optical information apparatusof Embodiment 20 according to the present invention.

FIG. 46 shows a state of a change in a diffraction efficiency in atangential direction of another exemplary hologram element of theoptical information apparatus of Embodiment 20 according to the presentinvention.

FIG. 47 shows a relationship between a light beam and division of stillanother exemplary hologram element of the optical information apparatusof Embodiment 20 according to the present invention.

FIG. 48 shows a state of a change in a diffraction efficiency in atangential direction of still another exemplary hologram element of theoptical information apparatus of Embodiment 20 according to the presentinvention.

FIG. 49 shows a relationship between a light beam and division of stillanother exemplary hologram element of the optical information apparatusof Embodiment 20 according to the present invention.

FIG. 50 shows a state of a change in a diffraction efficiency in atangential direction of still another exemplary hologram element of theoptical information apparatus of Embodiment 20 according to the presentinvention.

FIG. 51 shows a relationship between a light beam and division of ahologram element of an optical information apparatus of Embodiment 21according to the present invention.

FIG. 52 shows a state of a change in a diffraction efficiency in atangential direction of the hologram element of the optical informationapparatus of Embodiment 21 according to the present invention.

FIG. 53 shows a relationship between a light beam and division of aphotodetector of the optical information apparatus of Embodiment 21according to the present invention and a configuration of an electriccircuit.

FIG. 54 illustrates a configuration of an optical system of the opticalinformation apparatus of Embodiment 22 according to the presentinvention.

FIG. 55 shows a relationship between a light beam and division of aprism of the optical information apparatus of Embodiment 22 according tothe present invention.

FIG. 56 shows a state of a change in a diffraction efficiency in atangential direction of a prism of the optical information apparatus ofEmbodiment 22 according to the present invention.

FIG. 57 shows a relationship between a light beam and division of aphotodetector of the optical information apparatus of Embodiment 22according to the present invention and an electric circuit.

FIG. 58 illustrates a configuration of an optical system of the opticalinformation apparatus of Embodiment 23 according to the presentinvention.

FIG. 59 shows a relationship between a light beam and division of apolarizing hologram element of the optical information apparatus ofEmbodiment 23 according to the present invention.

FIG. 60 shows a relationship between a light beam and division of aphotodetector of the optical information apparatus of Embodiment 23according to the present invention and a configuration of an electriccircuit.

FIG. 61 shows a relationship between a light beam and division of apolarizing hologram element of an optical information apparatus ofEmbodiment 24 according to the present invention.

FIG. 62 shows a relationship between a light beam and division of aphotodetector of the optical information apparatus of Embodiment 24according to the present invention and a configuration of an electriccircuit.

FIG. 63 shows a configuration of an optical pick-up head constituting anoptical information apparatus of Embodiment 25 according to the presentinvention.

FIG. 64 shows a configuration of a beam splitter constituting an opticalinformation apparatus of Embodiment 25 according to the presentinvention.

FIG. 65 shows a relationship between a beam and a photodetectorconstituting an optical pick-up head in the optical informationapparatus of Embodiment 25 according to the present invention.

FIG. 66 shows a state of an amplitude with respect to an efficiency of0th order diffracted light in a region in the vicinity of a center of abeam splitter obtained in the optical information apparatus ofEmbodiment 25 according to the present invention.

FIG. 67 shows a configuration of a beam splitter constituting theoptical information apparatus of Embodiment 26 according to the presentinvention.

FIG. 68 shows a relationship between a beam and a photodetectorconstituting an optical information apparatus of Embodiment 27 accordingto the present invention.

FIG. 69 shows a configuration of a signal processing part constitutingthe optical information apparatus of Embodiment 27 according to thepresent invention.

FIG. 70 shows a configuration of an optical pick-up head constituting aconventional optical information apparatus.

FIG. 71 shows a state of a TE signal obtained in the conventionaloptical information apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an optical information apparatus, an optical pick-up head,and an optical information reproducing method of the present inventionwill be described by way of illustrative embodiments with reference tothe drawings. In each figure, the same reference numerals denote thesame components or those having similar functions and operations.

Embodiment 1

FIG. 1 shows a configuration of an optical information apparatus ofEmbodiment 1 according to the present invention. An optical pick-up head4 irradiates an optical recording medium 40 with laser light having awavelength λ of 405 nm, thereby reproducing a signal recorded on theoptical recording medium 40. A transport controller 5 moves the opticalpick-up head 4 in a radius direction of the optical recording medium 40so as to -record/reproduce information at any position on the opticalrecording medium 40. A motor 6 for driving the optical recording medium40 rotates the optical recording medium 40. A controller 7 controls theoptical pick-up head 4, the transport controller 5, and the motor 6.

An amplifier 8 amplifies a signal read by the optical pick-up head 4. Acontroller 9 receives an output signal from the amplifier 8. Based onthis signal, the controller 9 generates a servo signal such as a FE(focusing error) signal and a TE (tracking error) signal required forthe optical pick-up head 4 to read a signal of the optical recordingmedium 40 and outputs the servo signal to the controller 7. Furthermore,the controller 9 receives a signal in an analog form and digitizes it. Ademodulator 10 analyzes the digitized signal read from the opticalrecording medium 40, and reconfigures data of an original video andmusic. The reconfigured signal is output from an output unit 14.

A detector 11 detects an address signal and the like based on the signaloutput from the controller 9, and outputs the detected address signaland the like to a system controller 12. The system controller 12identifies the optical recording medium 40 based on physical formatinformation and optical recording medium production information (opticalrecording medium management information) read from the optical recordingmedium 40, and interprets a recording/reproducing condition and thelike, thereby controlling the entire optical information apparatus. Inthe case of recording/reproducing information with respect to theoptical recording medium 40, the controller 7 drives the transportcontroller 5 in accordance with an instruction from the systemcontroller 12. As a result, the transport controller 5 moves the opticalpick-up head 4 to a desired position on an information recording surface40 b (described later in FIG. 2) formed on the optical recording medium40. The optical pick-up head 4 records/reproduces information withrespect to the information recording surface 40 b of the opticalrecording medium 40.

FIG. 2 shows an exemplary configuration of the optical pick-up head 4according to Embodiment 1.

Alight source 1 emits a linearly polarized divergent beam 70 having awavelength λ of 405 nm. The divergent beam 70 emitted from the lightsource 1 is converted to parallel light by a collimator lens 53 having afocal length f1 of 15 mm, and thereafter, is incident upon a diffractiongrating 58. The beam 70 incident upon the diffraction grating 58 isdiffracted into three beams (0th order diffracted light and ±1st orderdiffracted light). The 0th order diffracted light functions as a mainbeam 70 a for recording/reproducing information, and the ±1st orderdiffracted light functions as two sub-beams 70 b and 70 c for adifferential push-pull (DPP) method for detecting a TE signal. The ratioof a diffraction efficiency between the 0th order diffracted light 70 aand the 1st order diffracted light 70 b or 70 c diffracted by thediffraction circuit 58 generally is set to be 10:1 to 20:1 so as toavoid unnecessary recording by the sub-beams. Herein, the ratio is setto be 20:1. Three beams 70 a to 70 c generated by the diffractiongrating 58 pass through a polarized beam splitter 52. Then, the beams 70a to 70 c pass through a quarter-wavelength plate 54 to be convertedinto circularly polarized light. Thereafter, the beams 70 a to 70 c areconverted to convergent beams by an objective lens 56 having a focallength f2 of 2 mm, and pass through a transparent substrate 40 a formedon the optical recording medium 40 to be condensed onto the informationrecording surface 40 b. An opening of the objective lens 56 is limitedby an aperture 55, and a numerical aperture NA is set to be 0.85. Thethickness of the transparent substrate 40 a formed on the opticalrecording medium 40 is 0.1 mm, and a refractive index n is 1.57.

FIG. 3 shows a relationship between a beam and a track on theinformation recording surface 40 b formed on the optical recordingmedium 40. The optical recording medium 40 is provided with a continuousgroove to be a track. Tn−1, Tn, and Tn+1 are tracks, respectively.Information is recorded on the groove to be a track. A pitch tp is 0.32μm. When the main beam 70 a is positioned on the track Tn, the sub-beam70 c is positioned between the track Tn−1 and the track Tn, and thesub-beam 70 b is positioned between the track Tn and the track Tn+1. Aninterval L in a direction orthogonal to each track between the main beam70 a and the sub-beams 70 b, 70 c is 0.16 μm.

The beams 70 a to 70 c reflected from the information recording surface40 b pass through the objective lens 56 and the quarter-wavelength plate54. Then, the beams 70 a to 70 c are converted to linearly polarizedlight whose plane of polarization is shifted by 90° from an ingoingpath, and thereafter, are reflected from the polarized beam splitter 52.The beams 70 a to 70 c reflected from the polarized beam splitter 52pass through a detection lens 59 having a focal length f3 of 30 mm and acylindrical lens 57 to be incident upon a photodetector 32. The beams 70a to 70 c are provided with astigmatism when passing through thecylindrical lens 57.

FIG. 4 schematically shows a relationship between the photodetector 32and the beams 70 a to 70 c incident upon the photodetector 32. Thephotodetector 32 has 8 light-receiving portions 32 a to 32 h in total.The light-receiving portions 32 a to 32 d arranged in a matrix receivethe beam 70 a, the light-receiving portions 32 e and 32 f receive thebeam 70 b, and the light-receiving portions 32 g and 32 h receive thebeam 70 c. The light-receiving portions 32 a to 32 h output currentsignals I32 a to I32 h in accordance with the received light amounts,respectively.

A FE signal is obtained by the astigmatism method using the signals I32a to I32 d output from the photodetector 32, i.e., by an arithmeticoperation (I32 a+I32 c)−(I32 b+I32 d). Furthermore, a TE signal isobtained by a DPP method, i.e., by an arithmetic operation {(I32 a+I32d)−(I32 b+I32 c)}−C·{(I32 e+I32 g)−(I32 f+I32 h)}. Herein, C is acoefficient determined by a ratio of a diffraction efficiency betweenthe 0th order diffracted light and one 1st order diffracted light of thediffraction grating 58. The FE signal and the TE signal are amplified toa desired level and compensated for phase. Thereafter, these signals aresupplied to actuators 91 and 92 for moving the objective lens 56,whereby focus and tracking control is performed.

FIG. 5 shows a TE signal according to the push-pull method obtained byscanning the beams 70 a to 70 c in a direction orthogonal to the tracksformed on the optical recording medium 40. Due to the error occurringmin the course of production of the optical recording medium 40, thetracks Tn−1 and Tn formed on the information recording surface 40 b ofthe optical recording medium 40 are formed at positions shifted by 25 nmfrom positions where they are supposed to be formed. Herein, the mainbeam 70 a and the sub-beams 70 b and 70 c are placed so as to be shiftedby tp/2 in a direction orthogonal to the tracks. Therefore, as shown inFIG. 5, when the main beam 70 a is positioned between the tracks Tn−1and Tn, and a signal with an amplitude S1 is obtained, the sub-beam 70 cis positioned between the tracks Tn−1 and Tn−2, and a signal with anamplitude S2 is obtained, and the sub-beam 70 b is positioned in thevicinity of the track Tn between the tracks Tn−1 and Tn, and a signalwith an amplitude S3 is obtained.

A signal obtained by averaging the signal with an amplitude S2 and thesignal with an amplitude S3 is a TE signal according to the push-pullmethod obtained from two sub-beams 70 b and 70 c. Herein, a relationship|(S2+S3)/2|>|S1| is satisfied. Assuming that the TE signal obtained fromthe main beam 70 a is a first push-pull signal, and the TE signalobtained from two sub-beams 70 b and 70 c is a second push-pull signal,according to the DPP method, a TE signal is obtained by subjecting thefirst push-pull signal and the second push-pull signal to a differentialarithmetic operation.

As described above, |(S2+S3)/2|>|S1| is satisfied. Therefore, afluctuation in a TE signal amplitude can be reduced by the DPP method.Regarding a TE signal obtained by a conventional optical informationapparatus described above, in the case where there is an error in aproduction position of tracks on the optical recording medium, the erroris reflected directly in the amplitude of the TE signal. In Embodiment1, positions different from that of the main beam 70 a in a directionorthogonal to tracks are irradiated with the sub-beams 70 b and 70 c.Therefore, even when the main beam 70 a is positioned on a track havingan error in a production position, the sub-beams 70 b and 70 c areplaced at other positions, so that the influence of this error isalleviated irrespective of the error in a production position when thetracks are formed on the optical recording medium 40. A fluctuationamount ΔPP of a TE signal is 0.69 in the conventional opticalinformation apparatus, whereas the fluctuation amount ΔPP in the opticalinformation apparatus of Embodiment 1 is 0.44, which is about ⅔ of thatof the conventional optical information apparatus. Thus, in the opticalinformation apparatus of Embodiment 1, a fluctuation in a TE signalamplitude is reduced, and a tracking operation can be performed stably.Therefore, information can be recorded/produced with high reliability.

Furthermore, in the conventional optical information apparatus, theoff-track oft1 in the track Tn−1 is +33 nm and the off-track oft2 in thetrack Tn is −33 nm. In the optical information apparatus according toEmbodiment 1, the off-track oft1 is +10 nm, and the off-track oft2 is−10 nm, which are about ⅓ of those in the conventional opticalinformation apparatus. Thus, in the optical information apparatus ofEmbodiment 1, even with the use of an inexpensive optical recordingmedium in which a TE signal amplitude is fluctuated, an off-track amountis small, and information recorded in adjacent tracks is unlikely to beerased. Accordingly, an optical information apparatus capable ofrecording/reproducing information with high reliability can be obtained.

Embodiment 2

FIG. 6 shows a relationship between a beam and a track on theinformation recording surface 40 b as an example of another opticalinformation apparatus according to the present invention. In the opticalpick-up head 4 constituting the optical information apparatus ofEmbodiment 1, when the main beam 70 a is positioned on the track Tn, thesub-beam 70 c is positioned between the tracks Tn−1 and Tn, and thesub-beam 70 b is positioned between the tracks Tn and Tn+1. In theoptical pick-up head constituting the optical information apparatus ofEmbodiment 2, when the main beam 70 a is positioned on the track Tn, thesub-beam 70 c is positioned between the tracks Tn−2 and Tn−1, and thesub-beam 70 b is positioned between the tracks Tn+1 and Tn+2. Morespecifically, an interval L in a direction orthogonal to the tracksbetween the main beam and the sub-beams is (3·tp)/2=0.48 μm. By slightlyrotating the diffraction grating 58 in the optical pick-up head 4, anoptical pick-up head constituting the optical information apparatus ofEmbodiment 2 can be configured. ATE signal can be obtained by thearithmetic operation similar to that described in Embodiment 1.

The interval L in a direction orthogonal to the tracks between the mainbeam and the sub-beams is set to be larger than that described withreference to FIG. 3 in Embodiment 1, whereby the fluctuation in a TEsignal amplitude can be reduced compared with the optical informationapparatus of Embodiment 1. The fluctuation amount ΔPP of the TE signalis 0.44 in the optical information apparatus of Embodiment 1, whereas inthe optical information apparatus of Embodiment 2, the fluctuationamount ΔPP of the TE signal is 0.21, which is about ½ of that ofEmbodiment 1. Thus, in the optical information apparatus of Embodiment2, the fluctuation in a TE signal amplitude is reduced further, and atracking operation can be performed stably, so that information can berecorded/reproduced with high reliability.

Furthermore, in the optical information apparatus of Embodiment 1, theoff-track oft1 in the track Tn−1 is +10 nm, and the off-track oft2 inthe track Tn is −10 nm. In the optical information apparatus ofEmbodiment 2, the off-track oft1 is −6 nm, and the off-track oft2 is +6nm, which is about ½ of that in Embodiment 1. Thus, in the opticalinformation apparatus of Embodiment 2, even in the case of using aninexpensive optical recording medium in which a TE signal amplitude isfluctuated, an off-track amount is reduced further, and informationrecorded on adjacent tracks is further unlikely to be erased. Thus, anoptical information apparatus is obtained, capable ofrecording/reproducing information with higher reliability.

Embodiment 3

FIG. 7 schematically shows a relationship between a photodetector 33 andthe beams 70 a to 70 c as an example of another optical informationapparatus according to Embodiment 3. By using the photodetector 33 inplace of the photodetector 32 constituting the optical pick-up head 4,an optical information apparatus of Embodiment 3 can be configured. Thephotodetector 33 has 12 light-receiving portions 33 a to 33 l in total.The light-receiving portions 33 a to 33 h receive the beam 70 a. Thelight-receiving portions 33 l to 33 j receive the beam 70 b. Thelight-receiving portions 33 k to 331 receive the beam 70 c. Thelight-receiving portions 33 a to 33 l output current signals I33 a toI33 l in accordance with the respectively received light amounts. A FEsignal is obtained by the astigmatism method using the current signalsI33 a to I33 h output from the photodetector 33, i.e., by an arithmeticoperation (I33 a+I33 b+I33 e+I33 f)−(I33 c+I33 d+I33 g+I33 h). Thisarithmetic operation seems to be complicated, since the photodetector 33has more light-receiving portions than the photodetector 32. Actually,this arithmetic operation is a general operation of obtaining a FEsignal by the astigmatism method.

On the other hand, a TE signal is obtained by a DPP method. The TEsignal herein is obtained by an arithmetic operation {(I33 a+I33 h)−(I33d+I33 e)}−C·{(I33 i+I33 k)−(I33 j+I33 l)}. Assuming that the TE signalobtained from the main beam 70 a is a first push-pull signal, and the TEsignal obtained from two sub-beams 70 b and 70 c is a second push-pullsignal, a TE signal is obtained by subjecting the first push-pull signaland the second push-pull signal to a differential operation in the sameway as in the optical information apparatus of Embodiment 1.

However, signals output from the light-receiving portions 33 b, 33 c, 33f, and 33 g that receive light in the vicinity of a center of the mainbeam 70 a are not used for generating the first push-pull signal.Furthermore, the light-receiving portions 33 i and 33 j that receive thesub-beam 70 b do not receive light in the vicinity of a center of thebeam 70 b. Herein, the width of a region that does not receive a beam isset to be 70% of the diameter of the beam. Similarly, thelight-receiving portions 33 k and 33 l that receive the sub-beam 70 c donot receive light in the vicinity of a center of the beam 70 c. Morespecifically, the first push-pull signal is generated without using theregion in the vicinity of the center of the main beam, and the secondpush-pull signal is generated without using the region in the vicinityof the center of the first and second sub-beams, which is different fromEmbodiments 1 and 2. This is based on the following principle: a numberof fluctuation components are concentrated in the vicinity of the centerof the beam when a track is formed by being fluctuated from a pitch tp.Therefore, the fluctuation can be reduced by avoiding the use of thevicinity of the center of the beam. For example, in the case where apositional shift of a track occurs every three tracks, three tracksshould be considered as one periodic structure. The pitch in this caseis three times of tp. The diffracted light from this periodic structurehas a long pitch, so that the diffraction angle of the beam is smalleraccordingly. That is, 1st order diffracted light from the periodicstructure is concentrated at the center of the beam.

The fluctuation in a TE signal amplitude can be reduced further,compared with the optical information apparatus of Embodiment 2. Thefluctuation amount ΔPP of the TE signal is 0.14 in the opticalinformation apparatus, which is ¼ or less of that of the conventionaloptical information apparatus. Thus, in the optical informationapparatus of Embodiment 3, the fluctuation in a TE signal amplitude canbe reduced, and a tracking operation can be performed stably. Therefore,the optical information apparatus of Embodiment 3 can record/reproduceinformation with high reliability.

Furthermore, in Embodiment 3, the off-track oft1 in the track Tn−1 is−11 nm, and the off-track oft2 in the track Tn is +11 nm, which areabout ⅓ of those of the conventional optical information apparatus.Thus, in the optical information apparatus of Embodiment 3, even in thecase of using an inexpensive optical recording medium in which a TEsignal amplitude is fluctuated, an off-track amount is further reduced,and information recorded on adjacent tracks is more unlikely to beerased. Thus, an optical information apparatus capable ofrecording/reproducing information with high reliability can be obtained.

The region in the vicinity of the center of the beam that is not usedfor generating a TE signal is prescribed to be a portion excluding aregion where 0th order diffracted light is overlapped with 1st orderdiffracted light from an optical recording medium in which a diffractionangle is dependent upon a pitch tp, a numerical aperture NA, and awavelength λ. Thus, the fluctuation in a TE signal amplitude can bereduced effectively.

Embodiment 4

FIG. 8 schematically shows a relationship between a photodetector 34 andthe beams 70 a to 70 c as an example of another optical informationapparatus according to the present invention. By using the photodetector34 in place of the photodetector 32 constituting the optical pick-uphead 4, an optical information apparatus of Embodiment 4 can beconfigured. The photodetector 34 has 16 light-receiving portions 34 a to34 p in total. The light-receiving portions 34 a to 34 h receive thebeam 70 a. The light-receiving portions 34 i to 34 j and 34 m to 34 nreceive the beam 70 b. The light-receiving portions 34 k to 34 l and 34o to 34 p receive the beam 70 c.

The light-receiving portions 34 a to 34 p output current signals I34 ato I34 p in accordance with the respectively received light amounts. AFE signal is obtained by the astigmatism method using the currentsignals I34 a to I34 h output from the photodetector 34. An arithmeticoperation is the same as that in the case of using the photodetector 33described with reference to FIG. 7.

On the other hand, a TE signal is obtained by a DPP method. The TEsignal herein is obtained by an arithmetic operation {(I34 a+I34 h)−(I34d+I34 e)}−K·{(I34 b+I34 g)−(I34 c+I34 f)}−C·[{(I34 i+I34 k)−(I34 j+I34l)}−k·{(I34 m+I34 o)−(I34 n+I34 p)}]. In this operation, k is acoefficient and a real number. Assuming that the TE signal obtained fromthe main beam 70 a is a first push-pull signal, and the TE signalobtained from two sub-beams 70 b and 70 c is a second push-pull signal,a TE signal is obtained by subjecting the first push-pull signal and thesecond push-pull signal to a differential operation according to the DPPmethod in the same way as in the optical information apparatus ofEmbodiment 1.

However, the above-mentioned operation is different from that accordingto the general DPP method in that, in order to generate the firstpush-pull signal, the signals output from the light-receiving portions34 b, 34 c, 34 f, and 34 g that receive light in the vicinity of thecenter of the main beam 70 a, the light-receiving portions 34 m and 34 nthat receive light in the vicinity of the center of the sub-beam 70 b,and the light-receiving portions 34 o and 34 p that receive light in thevicinity of the center of the sub-beam 70 c are multiplied by thecoefficient k, respectively. This is based on the following principle: anumber of fluctuation components are concentrated in the vicinity of thecenter of the beam when a track is formed by being fluctuated from apitch tp; therefore, the fluctuation can be reduced by manipulating thevicinity of the center of the beam. For example, in the case where apositional shift of a track occurs every three tracks, three tracksshould be considered as one periodic structure. The pitch in this caseis three times of tp. The diffracted light from this periodic structurehas a long pitch, so that the diffraction angle of the beam is smalleraccordingly. That is, 1st order diffracted light from the periodicstructure is concentrated at the center of the beam.

In the optical information apparatus of Embodiment 3, the fluctuation ina TE signal amplitude is suppressed by avoiding the use of the vicinityof the center of the beam. However, in Embodiment 4, the fluctuationcomponents mixed in the light-receiving portions 34 a, 34 d, 34 e, and34 h-34 l that detect a TE signal are cancelled with signals obtainedfrom the vicinity of the centers of the beams 70 a to 70 c received bythe light-receiving portions 34 b, 34 c, 34 f, 34 g, and 34 m-34 p,whereby the fluctuation in a TE signal amplitude is further reduced.

The fluctuation in a TE signal amplitude can be reduced further,compared with the optical information apparatus of Embodiment 2.Assuming that k=−0.45, the fluctuation amount ΔPP of the TE signal is0.28, the off-track oft1 in the track Tn−1 is 0 nm, and the off-trackoft2 in the track Tn is 0 nm. Thus, the fluctuation in a TE signalamplitude is reduced by ½ or less compared with the conventional opticalinformation apparatus, whereby an off-track can be reduced tosubstantially 0. More specifically, in the optical information apparatusof Embodiment 4, even in the case where a track position is shifted inthe course of production of an optical recording medium, information canbe recorded/reproduced with respect to the center of a groove at alltimes. On the other hand, assuming that k=0.35, the fluctuation amountΔPP of the TE signal is 0.04, the off-track oft1 in the track Tn−1 is−21 nm, and the off-track oft2 in the track Tn is +21 nm. Thefluctuation in a TE signal amplitude can be reduced to substantially 0,compared with the conventional optical information apparatus. Thus, theoptical information apparatus of Embodiment 4 has very stable trackingcontrol, and is capable of recording/reproducing information with highreliability. Furthermore, the off-tracks oft1 and oft2 represent a shiftamount from the center of a groove. Assuming that the off-track amountin the tracks Tn−1 and Tn are toft1 and toft2, respectively, in the casewhere information is always recorded on an optical recording medium at apitch tp, and tracks are virtually present at an interval of tp, theoff-track amount toft1 is +4 nm and the off-track amount toft2 is −4 nm,which are very small. That is, in the optical information apparatus ofEmbodiment 4, even in the case where the position of a groove is shiftedin the course of the production of the optical recording medium,information always can be recorded at a constant pitch, and informationrecorded on adjacent tracks is more unlikely to be erased. Thus, anoptical information apparatus capable of recording/reproducinginformation with high reliability can be obtained.

By splitting the beams 70 a to 70 c by the light-receiving portions 34 ato 34 p constituting the photodetector 34, the optical informationapparatus of the present embodiment can be configured without thenecessity of adding another optical component and without complicatingan optical system. Thus, an inexpensive optical information apparatuscan be provided.

Furthermore, the coefficient k for minimizing the fluctuation amount ofthe TE signal is different from that for minimizing an off-track.Therefore, in accordance with the performance required by the opticalinformation apparatus, the coefficient k may be set between the valuefor minimizing the fluctuation amount of the TE signal and the value forminimizing an off-track, whereby an optical information apparatus withgood balance of performance can be obtained.

Embodiment 5

FIG. 9 shows an exemplary configuration of an optical pick-up head 400of the present invention, as an example of another optical informationapparatus according to the present invention.

The optical pick-up head 400 is different from the optical pick-up head4 of Embodiment 1 in that a beam splitter 60 is provided between thepolarized beam splitter 52 and the condensing lens 59, and aphotodetector 35 is used in place of the photodetector 32. By using theoptical pick-up head 400 in place of the optical pick-up head 4, theoptical information apparatus of Embodiment 5 can be configured.

FIG. 10 schematically shows a configuration of the beam splitter 60. Thebeam splitter 60 has two kinds of regions 60 a and 60 b. The region 60 ais transparent, which transmits an incident beam as it is. On the otherhand, the region 60 b is provided with a blazed diffraction grating anddiffracts an incident beam efficiently in one direction. Thus, when thebeams 70 a to 70 c are incident upon the regions 60 a and 60 b, thebeams 70 a to 70 c are split into two, respectively.

FIG. 11 schematically shows a relationship between the photodetector 35and the beams 70 a to 70 c. The photodetector 35 has 16 light-receivingportions 35 a to 35 p in total. The light-receiving portions 35 a to 35h receive the beams 70 a to 70 c having passed through the region 60 aof the beam splitter 60, and the light-receiving portions 35 i to 35 preceive the beams 70 a to 70 c diffracted in the region 60 b of the beamsplitter 60. Sixteen light-receiving portions 35 a to 35 p outputcurrent signals 135 a to 135 p in accordance with the respectivelyreceived light amounts. A FE signal is obtained by an arithmeticoperation (I35 a+I35 c+I35 i+I35 k)−(I35 b+I35 d+I35 j+I35 l). Thisarithmetic operation seems to be complicated, since the photodetector 35has more light-receiving portions than the photodetector 32. Actually,this arithmetic operation is a general operation of obtaining a FEsignal by the astigmatism method.

A TE signal is obtained by a DPP method. The TE signal herein isobtained by an arithmetic operation {(I35 a+I35 d)−(I35 b+I35c)}−C·{(I35 e+I35 g)−(I35 f+I35 h)}−k·[{(I35 i+I35 l)−(I35 j+I35k)}−C·{(I35 m+I35 o)−(I35 n+I35 p)}].

The characteristics of a TE signal to be obtained are the same as thoseof the optical information apparatus of Embodiment 4. The TE signal alsomay be obtained by an arithmetic operation {(I35 a+I35 d)−(I35 b+I35c)}−k·{(I35 e+I35 g)−(I35 f+I35 h)}. The characteristics of the TEsignal are the same as those of the optical information apparatus ofEmbodiment 3.

On the other hand, in the optical information apparatus of Embodiment 5,a spherical aberration error signal that is indicative of the sphericalaberration amount of the beams 70 a to 70 c condensed onto the opticalrecording medium 40 can be generated. The spherical aberration errorsignal is obtained by an arithmetic operation (I35 a+I35 c)−(I35 b+I35d)}−C2·−{(I35 i+I35 k)−(I35 j+I35 l)}. More specifically, signals outputfrom the light-receiving portions 35 i to 35 l receiving a region in thevicinity of the center of the main beam 70 a are subjected to adifferential operation to generate a first FE signal. Signals outputfrom the light-receiving portions 35 a to 35 d receiving a region in thevicinity of the outside of the main beam 70 a are subjected to adifferential operation to generate a second FE signal. Then, the firstFE signal and the second FE signal are subjected to a differentialoperation to obtain a spherical aberration error signal. Herein, acoefficient C2 is a real number, which is a correction coefficient foradjusting the spherical aberration error signal to be 0 at a desiredspherical aberration amount. A spherical aberration corrector isprovided in the optical pick-up head 4, and is controlled using aspherical aberration error signal. Because of this, a sphericalaberration of a beam condensed onto the optical recording medium 40 canbe reduced, and a mark with less jitter can be recorded on the opticalrecording medium, whereby an optical information apparatus with highreliability can be provided. The spherical aberration corrector can havea generation configuration such as a liquid crystal element, a concaveand convex cemented lens, and the like. Therefore, the descriptionthereof is omitted here.

Embodiment 6

FIG. 12 shows an exemplary configuration of an optical pick-up head 401according to the present invention, as an example of another opticalinformation apparatus according to the present invention.

The difference between the optical pick-up head 4 in Embodiment 1 andthe optical pick-up head 401 in Embodiment 6 is as follows. In theoptical pick-up head 401, the diffraction grating 58 is not used, sothat one beam 70 is condensed onto the information recording surface 40b of the optical recording medium 40. Furthermore, a beam splitter 61 isprovided. The beam splitter 61 is integrated with the quarter-wavelengthplate 54 and the objective lens 56. The actuators 91 and 92 drive thebeam splitter 61, the quarter-wavelength plate 54, and the objectivelens 56 to perform focus control and tracking control. The beam splitter61 is dependent upon polarization, and transmits all the incident beams70 in an ingoing path toward the optical recording medium 40 from thelight source 1. On the other hand, in an outgoing path toward thephotodetector 36 of a beam reflected from the optical recording medium40, a large part of the incident light beam is transmitted, and a partthereof is diffracted to generate a plurality of diffracted light beams.Furthermore, the photodetector 36 is used in place of the photodetector32. By using the optical pick-up head 401 in place of the opticalpick-up head 4, an optical information apparatus of Embodiment 6 can beconfigured.

FIG. 13 schematically shows a configuration of the beam splitter 61. Thebeam splitter 61 has 4 kinds of regions 61 a to 61 d, which transmit alarge part of the incident beam 70 to generate a 0th order diffractedlight beam 710 and diffract a part of the incident beam 70 to generatethe beams 71 a to 71 d′from the regions 61 a to 61 d, respectively.

FIG. 14 schematically shows a relationship between the photodetector 36and the beams 71 a to 71 d, and 710. The photodetector 36 has 8light-receiving portions 36 a to 36 h in total. The light-receivingportions 36 a to 36 d receive the beam 710. The light-receiving portion36 g receives the beam 71 a. The light-receiving portion 36 e receivesthe beam 71 b. The light-receiving portion 36 f receives the beam 71 c.The light-receiving portion 36 h receives the beam 71 d. Thelight-receiving portions 36 a to 36 h output current signals I36 a toI36 h in accordance with the respectively received light amounts. A FEsignal is obtained by an arithmetic operation (I36 a+I36 c)−(I36 b+I36d).

On the other hand, a TE signal is obtained by an arithmetic operation(I36 g−I36 h)−k·(I36 e−I36 f). Assuming that k=0.35, a fluctuationamount ΔPP of the TE signal is 0.04, an off-track oft1 in the track Tn−1is −19 nm, and an off-track oft2 in the track Tn is +19 nm. Thus, thefluctuation in a TE signal amplitude can be reduced to substantially 0compared with the conventional optical information apparatus, andtracking control can be performed very stably.

The TE signal also may be obtained by an arithmetic operation (I36 g−I36h). At this time, the fluctuation amount ΔPP of the TE signal is 0.24,the off-track oft1 in the track Tn−1 is −1 nm, and the off-track oft2 inthe track Tn is +1 nm. Thus, even in the case where the position of atrack is shifted in the course of production of the optical recordingmedium, information can be recorded at the center of a groove at alltimes.

In the optical information apparatus of Embodiment 6, only one beam 71is condensed onto the optical recording medium 40. Therefore, even inthe case where the optical recording medium 40 has large eccentricity,the fluctuation amount of the TE signal amplitude is not increased, andtracking control can be performed stably.

Furthermore, the beam splitter 61, the quarter-wavelength plate 54, andthe objective lens 56 are integrated, and driven by the actuators 91 and92. Even in the case where a track is followed while the opticalrecording medium 40 has eccentricity, the position at which the beam 71is split is always constant. Therefore, the fluctuation in a TE signalamplitude always can be reduced stably without depending upon the amountof eccentricity of the optical recording medium 40. Furthermore, thesplit width of the beam 71 can be set so as to minimize the fluctuationin a TE signal amplitude without considering the eccentricity of theoptical recording medium, thereby providing an optical informationapparatus capable of further reducing the fluctuation in a TE signalamplitude. Furthermore, an offset occurring in a TE signal when a trackis followed can be reduced.

Furthermore, since the diffraction grating 58 is not provided, an outputamount from the light source 1 required for recording information on theoptical recording medium 40 may be smaller than that of the opticalpick-up head 4. This reduces the burden of the light source 1accordingly, and prolongs the life of the light source 1. Thus, anoptical information apparatus that can be used for a long period of timeis provided.

Furthermore, the regions 61 a to 61 d of the beam splitter 61 have alens effect so that diffracted light is focused on the photodetector 36,whereby the size of the light-receiving portions 36 e to 36 h can bereduced. As the size of the light-receiving portions 36 e to 36 h issmaller, they are less likely to be influenced by stray light, whichallows tracking control to be performed more stably. This is effective,particularly, in the case of using an optical recording medium having aplurality of information recording surfaces. When the size of thelight-receiving portions is decreased, even if the focal length of thecondensing lens 59 is shortened, i.e., even if the magnification of adetection optical system is decreased, the influence of stray light isnot increased, enabling an optical information apparatus stable to achange with passage of time to be provided.

Embodiment 7

FIG. 15 schematically shows a relationship between a photodetector 37and the beams 71 b to 71 c, and 710, as an example of another opticalinformation apparatus according to the present invention. By using thephotodetector 37 in place of the photodetector 36 in Embodiment 6, anoptical information apparatus of Embodiment 7 can be configured. Thephotodetector 37 is obtained by eliminating the light-receiving portions36 g and 36 h from the photodetector 36. The photodetector 37 has 6light-receiving portions 37 a to 37 f in total. The light-receivingportions 37 a to 37 d receive the beam 710. The light-receiving portion37 e receives the beam 71 b. The light-receiving portion 37 f receivesthe beam 71 c.

A TE signal is obtained by an arithmetic operation {(I37 a+I37 d)−(I37b+I37 c)}−k·(I37 e−I37 f). By appropriately selecting a coefficient k,the same characteristics as those of the optical information apparatusof Embodiment 6 can be obtained. The photodetector 37 is smaller thanthe photodetector 36 of Embodiment 6, so that an optical pick-up head ofthe present embodiment becomes smaller than that of Embodiment 6 by thedecreased size of the photodetector 37. Furthermore, the number oflight-receiving portions of the photodetector 37 is smaller than that ofthe photodetector 36, so that a circuit for processing a signal also isdecreased in size and becomes less expensive.

Furthermore, since diffracted light may not be generated from theregions 61 a and 61 d of the beam splitter, if a light beam is allowedsimply to pass through the beam splitter without forming the regions 61a and 61 d, the light amount of the beam 710 is increased accordingly.Therefore, a S/N ratio is enhanced at a time when information recordedon the optical recording medium 40 is read.

Embodiment 8

FIG. 16 schematically shows a configuration of a beam splitter 62, as anexample of another optical information apparatus according to thepresent invention. By using the beam splitter 62 in place of the beamsplitter 61 in Embodiment 6 and using a photodetector 38 in place of thephotodetector 36, an optical information apparatus of Embodiment 8 canbe configured. The beam splitter 62 has two kinds of regions 62 a and 62b. The beam splitter 62 transmits a large part of the incident beam 70to generate a 0th order diffracted light beam 710, and diffracts a partthereof to generate beams 73 a to 73 b from the regions 62 a to 62 b,respectively.

FIG. 17 schematically shows a relationship between the photodetector 38and the beams 73 a, 73 b, and 710.

The photodetector 38 has 12 light-receiving portions 38 a to 38 l intotal. The light-receiving portions 38 a to 38 d receive the beam 710.The light-receiving portions 38 e to 38 h receive the beam 73 a. Thelight-receiving portions 38 i to 38 l receive the beam 73 b. Thelight-receiving portions 38 a to 381 output current signals I38 a to I38l in accordance with the respectively received light amounts. A FEsignal is obtained by an arithmetic operation (I38 a+I38 c)−(I38 b+I38d).

A TE Signal is obtained by an arithmetic operation (I38 e+I38 h)−(I38f+I38 g). The TE signal also may be obtained by an arithmetic operation{(I38 e+I38 h)−(I38 f+I38 g)}−k·{(I38 i+I38 l)−(I38 j+I38 k)}. In thecase where the beam splitter 62 is integrated with the objective lens56, either of the arithmetic operations may be used. However, in thecase where the beam splitter 62 is not integrated with the objectivelens 56, the latter arithmetic operation preferably is used for thefollowing reason. According to the latter arithmetic operation, in thecase where an actuator is moved when a track is followed, an offsetoccurring in the TE signal becomes smaller compared with that accordingto the former arithmetic operation.

A spherical aberration error signal is obtained by an arithmeticoperation (I38 e+I38 g)−(I38 f+I38 h)}−C2·{(I38 i+I38 k)−(I38 j+I38 l)}.In the optical information apparatus of Embodiment 8, the fluctuation ina TE signal amplitude can be reduced in the same way as in the opticalinformation apparatus described in Embodiment 5. The quality of thespherical aberration error signal is more satisfactory than that in theoptical information apparatus of Embodiment 5. A spherical aberrationcan be corrected more precisely, and a mark with less jitter can berecorded on an optical recording medium. Thus, an optical informationapparatus with high reliability can be provided.

Furthermore, it also may be possible that the beam splitter 62 is placedin an optical path leading from the polarized beam splitter 52 to thephotodetector 38 without being integrated with the objective lens 56. Inthis case, it is not necessary that the beam splitter 62 haspolarization dependency, and it may be of a non-polarization type. Thebeam splitter 62 can be produced by very inexpensive resin molding, sothat an inexpensive optical information apparatus can be provided.

Embodiment 9

FIG. 18 schematically shows a configuration of a beam splitter 63, as anexample of another optical information apparatus according to thepresent invention. By using the beam splitter 63 in place of the beamsplitter 61 in Embodiment 6 and using the photodetector 39 in place ofthe photodetector 36, an optical information apparatus according toEmbodiment 9 can be configured. The beam splitter 63 has three kinds ofregions 63 a to 63 c. The beam splitter 63 transmits a large part of theincident beam 70 to generate a 0th order diffracted light beam 710, anddiffracts a part thereof to generate beams 74 a to 74 c from the regions63 a to 63 c, respectively.

FIG. 19 schematically shows a relationship between the photodetector 39and the beams 74 a to 74 c, and 710. The photodetector 39 has 16light-receiving portions 39 a to 39 p in total. The light-receivingportions 39 a to 39 d receive the beam 710. The light-receiving portions39 e to 39 h receive the beam 74 a. The light-receiving portions 39 i to39 l receive the beam 74 b. The light-receiving portions 39 m to 39 preceive the beam 74 c. The light-receiving portions 39 a to 39 p outputcurrent signals 139 a to 139 p in accordance with the respectivelyreceived light amounts. A FE signal is obtained by an arithmeticoperation (I39 a+I39 c)−(I39 b+I39 d).

A TE signal is obtained by an arithmetic operation (I39 m+I39 p)−(I39n+I39 o). The TE signal also may be obtained by an arithmetic operation{(I39 m+I39 p)−(I39 n+I39 o)}−k·{(I39 e+I39 h)−(I39 f+I39 g)}. The TEsignal also may be obtained by an arithmetic operation {(I39 m+I39p)−(I39 n+I39 o)}−k{(I39 i+I39 l)−(I39 j+I39 k)}. The TE signal also maybe obtained by an arithmetic operation {(I39 m+I39 p)−(I39 n+I39o)}k·{(I39 e+I39 g+I39 i+139 l)−(I39 f+I39 g+I39 j+I39 k)}. According toeither of the arithmetic operations, the fluctuation in a TE signalamplitude can be reduced. In the case where the beam splitter 63 isintegrated with the objective lens 56, either of the arithmeticoperations may be used. However, in the case where the beam splitter 63is not integrated with the objective lens 56, the second to fourtharithmetic operations preferably are used. According to the second tofourth arithmetic operations, in the case where an actuator is movedwhen a track is followed, an offset occurring in a TE signal is smallerthan that according to the arithmetic operation. In the case of usingeither the first arithmetic operation or the fourth arithmeticoperation, the same characteristics as those of the optical informationapparatus of Embodiment 6 can be obtained. According to the secondarithmetic operation, even in the case where defocusing occurs, anoptical information apparatus can be provided, which has less off-trackand has high reliability with respect to external disturbance such asdefocusing and the like.

A spherical aberration error signal is obtained by an arithmeticoperation (I39 i+I39 k)−(I39 j+I39 l)}−C2·{(I39 e+I39 g+I39 m+I39o)−(I39 f+I39 h+I39 n+I39 p)}. Furthermore, the quality of the sphericalaberration error signal in the present embodiment is more satisfactorythan that of the optical information apparatus of Embodiment 5. Aspherical aberration can be corrected more precisely, and a mark withless jitter can be recorded on an optical recording medium. Thus, anoptical information apparatus with high reliability can be provided.

In the above-mentioned embodiments, the width of a beam in the vicinityof the center is set to be 0.7 times the diameter of the beam so as tosuppress the fluctuation in a TE signal amplitude. The reason for thisis that all the embodiments are put under the same condition so that ΔPPcan be compared with the improvement of off-track. Thus, there is noparticular limit to the width of the beam in the vicinity of the center,and it can be set freely. It also is appreciated that a beam is notnecessarily split by a straight line.

The case where the fluctuation in a TE signal amplitude occurs due to apositional error during formation of a groove has been described.However, the fluctuation in a TE signal amplitude occurs similarly evenin the case where there is an error in the width and depth of a groove,and even in the vicinity of a boundary between a track of an opticalrecording medium in which information is recorded and an unrecordedtrack. The present invention also is applicable to these cases.

Embodiment 10

FIG. 20 shows an exemplary configuration of an optical pick-up head 402according to the present invention, as an example of another opticalinformation apparatus according to the present invention.

The optical pick-up head 402 of Embodiment 10 is different from theoptical pick-up head 400 of Embodiment 5 in the following points: thediffraction grating 58 is not used; an optical recording medium 41 hastwo information recording layers 41 b and 41 c; and a beam splitter 64is used in place of the beam splitter 60 and a photodetector 45 is usedin place of the photodetector 35. Since the diffraction grating 58 isnot used, one beam 70 output from the light source 1 is condensed ontothe information recording surface of the optical recording medium 41.

The optical recording medium 41 has two information recording surfaces41 b and 41 c. Herein, the beam 70 condensed by the objective lens 56 isshown as being focused on the information recording surface 41 c. Theoptical recording medium 41 is composed of a transparent substrate 41 aand information recording surfaces 41 b and 41 c. A distance d2 from thelight incident surface of the optical recording medium 41 to theinformation recording surface 41 c is 100 μm, and an interval d1 betweenthe information recording surfaces 41 b and 41 c is 25 μm. Although notshown here, a pitch tp of tracks formed on the information recordingsurfaces 41 b and 41 c is 0.32 μm.

A wavelength λ of the light source 1 is 405 nm, and a numerical apertureof the objective lens is 0.85. The equivalent reflectance of theinformation recording surfaces 41 b and 41 c is about 4 to 8%. Herein,the equivalent reflectance refers to the light amount of a beam that isreflected from the information recording surface 41 b or 41 c and outputfrom the optical recording medium 41, assuming that the light amount ofa beam incident upon the optical recording medium 41 is 1. Theinformation recording surface 41 c absorbs or reflects a large part ofthe light amount of an incident beam, whereas the information recordingsurface 41 b transmits about 50% of the light amount of an incident beamso as to allow the beam to reach the information recording surface 41 cand absorbs or reflects the remaining 50% of the light amount.

The beam 70 reflected from the information recording surface 41 c of theoptical recording medium 41 passes through the objective lens 56, isreflected from the polarized beam splitter 52, and is incident upon thebeam splitter 64. The beam splitter 64 generates a plurality of beams75. The plurality of beams 75 generated by the beam splitter 64 passthrough the cylindrical lens 57 to be provided with astigmatism, and arereceived by the photodetector 45.

FIG. 21 schematically shows a configuration of the beam splitter 64, andFIG. 22 schematically shows a relationship between the photodetector 45and the beam 75 received by the photodetector 45. The beam splitter 64has 7 kinds of regions 64 a to 64 g in total. In the beam splitter 64, Ddenotes a diameter of the beam 70 incident upon the beam splitter 64after being reflected from the polarized beam splitter 52, and generallyis set to be about 2 to 4 mm. Herein, D is set to be 3 mm.

The beam 75 contains a 0th order diffracted light beam 75 a and 7 1storder diffracted light beams 75 b to 75 h. The beam splitter 64 is akind of a diffraction grating. Herein, the diffraction efficiency of the0th order diffracted light is set to be 80%, and the diffractionefficiency of the 1st order diffracted light is set to be 8%. The reasonfor setting the diffraction efficiency of the 0th order diffracted lightto be higher than that of the 1st order diffracted light is thatinformation recorded on the information recording surfaces 41 b and 41 cof the optical recording medium 41 is read using 0th order diffractedlight, and 1st order diffracted light is used only for generating atracking error signal. As the diffraction efficiency of 0th orderdiffracted light is larger, a S/N ratio at a time when informationrecorded on the information recording surfaces 41 b and 41 c is read canbe enhanced. Therefore, information can be reproduced faithfully.

The beam 75 a is generated from the regions 64 a to 64 g. The beam 75 bis generated from the region 64 a. The beam 75 c is generated from theregion 64 b. The beam 75 d is generated from the region 64 c. The beam75 e is generated from the region 64 d. The beam 75 f is generated fromthe region 64 e. The beam 75 g is generated from the region 64 f. Thebeam 75 h is generated from the region 64 g. Patterns formed in theregions 64 a to 64 g are simple gratings in a linear shape with an equalpitch. The beam 70 is moved in a direction represented by an arrow TRKon the beam splitter 64 in accordance with tracking control.

By forming the regions 64 a to 64 f sufficiently larger than a radius rof the beam 70, a TE signal is prevented from decreasing during trackingcontrol. Herein, a size h3 of the regions 64 a to 64 f in the directionrepresented by the arrow TRK is set to be larger than the radius r ofthe beam 70 by 500 μm. On the other hand, the size of the regions 64 ato 64 f in the direction orthogonal to the arrow TRK representing atrack-following direction only needs to be a positional shift tolerancebetween the beam 70 and the beam splitter 64 in the course of assemblyof an optical pick-up head. Therefore, this size generally may be 10 to100 μm, and herein, a-width h4 is set to be larger than the diameter Dof the beam 70 by 100 μm. A width h1 is set to be 0.35D, and a width h2is set to be 0.6 D.

Referring to FIG. 22, the photodetector 45 has 10 light-receivingportions 45 a to 45 j in total. The light-receiving portions 45 a to 45d are used for detecting a FE signal and a signal for reproducinginformation recorded on the optical recording medium 41, and thelight-receiving portions 45 e to 45 j are used for detecting a TEsignal. By forming the light-receiving portions 45 a to 45 d fordetecting a FE signal and the light-receiving portions 45 e to 45 j fordetecting a TE signal on the same semiconductor substrate, an opticalpick-up head can be miniaturized, and the number of steps of assemblingthe optical pick-up head can be decreased.

The beam 75 a is received by the four light-receiving portions 45 a to45 d. The beam 75 b is received by the light-receiving portion 45 e. Thebeam 75 c is received by the light-receiving portion 45 f. The beam 75 dis received by the light-receiving portion 45 g. The beam 75 e isreceived by the light-receiving portion 45 h. The beam 75 f is receivedby the light-receiving portion 45 i. The beam 75 g is received by thelight-receiving portion 45 j. The beam 75 h is designed so as not to bereceived by any of the light-receiving portions. Because of thisconfiguration, in the same way as in Embodiment 3 and the like, afluctuation in a TE signal is reduced, which occurs when there is avariation in a position, a width, and a depth of a groove formed on anoptical recording medium and when information is recorded in a track.

Furthermore, the above-mentioned configuration also has a function ofpreventing unnecessary light from being incident upon a light-receivingportion used for detecting a TE signal, in the case where an opticalrecording medium has a plurality of information recording surfaces. Thelight-receiving portions 45 a to 45 j output current signals I45 a toI45 j in accordance with the respectively received light amounts. A FEsignal is obtained by an arithmetic operation (I45 a+I45 c)-(I45 b+I45d). A method for detecting a TE signal will be described later.

The beams 75 a to 75 h are generated when the beam 70 reflected from theinformation recording surface 41 c is incident upon the beam splitter64. The optical recording medium 41 has two information recordingsurfaces 41 b and 41 c. Therefore, a beam reflected from the informationrecording surface 41 b also is incident upon the beam splitter 64 afterbeing reflected from the polarized beam splitter 52, whereby diffractedlight is generated in the beam splitter 64. The beams 76 a to 76 h arediffracted light generated when the beam 70 reflected from theinformation recording surface 41 b is incident upon the beam splitter64. The beam 76 a is generated from the regions 64 a to 64 g. The beam76 b is generated from the region 64 a. The beam 76 c is generated fromthe region 64 b. The beam 76 d is generated from the region 64 c. Thebeam 76 e is generated from the region 64 d. The beam 76 f is generatedfrom the region 64 e. The beam 76 g is generated from the region 64 f.The beam 76 h is generated from the region 64 g.

The beam 70 is focused on the information recording surface 41 c, sothat the beam 70 is defocused largely on the information recordingsurface 41 b. Therefore, the beams 76 a to 76 h also are defocusedlargely on the photodetector 45. Herein, the beams 76 a to 76 h areprevented from being incident upon the light-receiving potions 45 e to45 j. The reason for this is as follows: when the beams 76 a to 76 h areincident upon the light-receiving portions 45 e to 45 j, a TE signal isdisturbed in accordance with the incident degree; consequently, stabletracking control cannot be performed. Herein, by forming thelight-receiving portions 45 e to 45 j at a position away from the radiusof the beam 76 a, the beam 76 a is prevented from being incident uponthe light-receiving portions 45 e to 45 j.

Furthermore, the light-receiving portions 45 e to 45 j are formed closeto each other so as to receive the beams 75 b to 75 g. Furthermore, aregion 64 g is provided at the center of the beam splitter 64 shown inFIG. 21, and the beam 75 h generated from the region 64 g is not usedfor generating a TE signal. Because of this arrangement, the beams 76 bto 76 g are positioned outside of the light-receiving portions 45 e to45 j, i.e., the beams 76 b to 76 g are not incident upon thelight-receiving portions 45 e to 45 j. Furthermore, the beam 75 h′isdiffracted in a direction orthogonal to the beams 75 b to 75 g. Becauseof this, the light-receiving portions 45 e to 45 j are formed atpositions where the beam 76 a is not incident, whereby the beam 76 h isnot incident upon the light-receiving portions 45 e to 45 j withoutfail.

Furthermore, the beam 76 a is moved in the direction represented by thearrow TRK in accordance with tracking control. By forming thelight-receiving portions 45 e to 45 j in a direction orthogonal to thedirection represented by the arrow TRK, the beam 76 a is prevented frombeing incident upon the light-receiving portions 45 e to 45 j bytracking control, and the light-receiving portions 45 e to 45 j can beformed dose to the light-receiving portions 45 a to 45 d. Because ofthis, the photodetector is decreased in size, and an inexpensive opticalpick-up head can be provided.

As the condition for preventing the beams 76 b to 76 g from beingincident upon the light-receiving portions 45 e to 45 j, a width W1 ofthe light-receiving portions 45 e to 45 j only needs to be2·h1/D·d1/n2·αNA or less. Similarly, a width W2 of the light-receivingportions 45 e to 45 j only needs to be 2·h2/D·d1/n2·α·NA or less, whered1 is an interval between the information recording surfaces 41 b and 41c; n2 is a refractive index of a medium present between the informationrecording surfaces 41 b and 41 c of the optical recording medium 41; αis a lateral magnification of an optical system from the opticalrecording medium 41 to the photodetector 45; D is a diameter of the beam70 on the beam splitter 64; h1 and h2 are widths of the region 64 g ofthe beam splitter 64; and NA is a numerical aperture of the objectivelens 56. Herein, the lateral magnification α is about f3/f2. The widthsh1 and h2 of the region 64 g of the beam splitter 64 and the widths W1and W2 of the light-receiving portions 45 e to 45 j are set so as tosatisfy the above condition. It is appropriate that the lateralmagnification of the optical system is about 4 to 40 times.

Herein, the case where the optical recording medium 41 has twoinformation recording surfaces has been described. However, the presentinvention similarly is applicable to the case where the opticalrecording medium has three or more information recording surfaces. Ingeneral, an antireflection film is not formed on a light incidentsurface (hereinafter, briefly referred to as a “surface”) of an opticalrecording medium. Therefore, a beam travels to a photodetector afterbeing reflected from the surface of the optical recording medium. Thebeam reflected from the surface of the optical recording medium maycause tracking control to be unstable. Therefore, it is desirable thatthis beam is not incident upon the light-receiving portions 45 e to 45j. It also is possible that the beam reflected from the surface of theoptical recording medium is prevented from being incident upon thelight-receiving portions 45 e to 45 j by the above-mentioned designmethod. More specifically, by setting an interval d between a desiredinformation recording surface and another surface for reflecting a beamsuch as the surface of the optical recording medium, instead of settingthe interval d1 between the information recording surfaces 41 b and 41c, the beam reflected from the surface of the optical recording mediumis allowed to be incident upon an arbitrary surface.

FIG. 23 shows a configuration of a signal processing portion forgenerating a TE signal. Signals I45 e and I45 f output from thelight-receiving portions 45 e and 45 f are subjected to a differentialarithmetic operation in a differential arithmetic operation portion 801.A signal (I45 f−145 e) obtained by a differential arithmetic operationis a TE signal according a so-called push-pull method.

The beam splitter 64 is not integrated with the objective lens 56.Therefore, when the objective lens 56 is subjected to track-following inaccordance with the eccentricity of the optical recording medium 41, anoffset fluctuation is caused in a TE signal in accordance withtrack-following. In the signal processing portion shown in FIG. 23,signals I45 g and I45 h output from the light-receiving portions 45 gand 45 h are added to each other in an adder 802, and signals I451 andI45 j output from the light-receiving portions 45 i and 45 j are addedto each other in an adder 803. The signals output from the adders 802and 803 are subjected to a differential arithmetic operation in thedifferential arithmetic operation portion 804. The signal output fromthe differential arithmetic operation portion 804 is input to a variablegain amplifier 805, and is amplified or attenuated to a desired signalintensity. Herein, the signal output from the differential arithmeticoperation 804 is provided with an amplification degree of about 2.5. Thesignal output from the variable gain amplifier 805 has the samefluctuation as an offset fluctuation of the signal output from thedifferential arithmetic operation portion 801. The offset fluctuationoccurs when a tracking operation, an access operation, or the like isperformed with respect to an optical recording medium havingeccentricity.

The differential arithmetic operation portion 806 receives the signaloutput from the differential arithmetic operation portion 801 and thesignal output from the variable gain amplifier 805 and subjects them toa differential arithmetic operation, whereby the above-mentioned offsetfluctuation of the signal output from the differential arithmeticoperation portion 801 is reduced. The signal output from thedifferential arithmetic operation portion 806 is a TE signal havingsubstantially no offset fluctuation even if a track is followed.However, if the signal output from the differential arithmetic operationportion 806 is used as it is, the signal intensity is changed inaccordance with the reflectance of the information recording surfaces 41b and 41 c of the optical recording medium 41 and the change in anintensity of a beam radiated to the optical recording medium 41.Therefore, the signal is input to a divider 808 so as to have a constantamplitude.

The signals I45 a to I45 d output from the light-receiving portions 45 ato 45 d are added to each other in the adder 807, and input to thedivider 808 as a signal to be divided. The signal output from the adder807 is proportional to the reflectance of the information recordingsurfaces 41 b and 41 c of the optical information medium 41 and theintensity of a beam radiated to the optical recording medium 41. Thesignal output from the divider 808 is a TE signal having a desiredintensity.

In an optical information apparatus using the optical pick-up head andthe signal processing portion according to Embodiment 10, even in thecase where an optical recording medium has a plurality of informationrecording surfaces, a stable tracking operation is possible. Thus, theoptical information apparatus is highly reliable. Furthermore, the beams75 d to 75 g are received by the light-receiving portions 45 g to 45 j.Therefore, by comparing the intensities of the signals output from thelight-receiving portions 45 g to 45 j, the set position of the beamsplitter 64 with respect to the beam 70 can be recognized easily.Therefore, the beam splitter 64 easily can be set with respect to thebeam 70 with good precision, whereby the productivity of an opticalpick-up head can be enhanced.

Similarly to the optical pick-up head in Embodiment 6, it may bepossible that the beam splitter 64 is composed of apolarization-dependent element and integrated with the objective lens56. In this case, the position of the beam 70 on the beam splitter 64 isalways constant. Therefore, even if the width h1 of the region 64 g isincreased, the amplitude of a TE signal is not decreased. Because ofthis, the beams 76 d to 76 g are unlikely to be incident upon thelight-receiving portions 45 e to 45 j accordingly.

Embodiment 11

FIG. 24 schematically shows an exemplary relationship between aphotodetector 46 constituting the optical pick-up head of the presentinvention and the beams 75 a to 75 h and 76 a to 76 h, as an example ofanother optical information apparatus according to the presentinvention. By using the photodetector 46 in place of the photodetector45 in the optical pick-up head 402 in Embodiment 10, and slightlychanging the patterns to be formed in the regions 64 d and 64 f of thebeam splitter 64, an optical pick-up head in Embodiment 11 can beconfigured.

In the optical pick-up head in the present embodiment, the beams 75 dand 75 e are received by one light-receiving portion 46 g, and the beams75 f and 75 g are received by one light-receiving portion 46 h.Furthermore, the beams 75 d and 75 e are overlapped with each other onthe light-receiving portion 46 g, and the beams 75 f and 75 g areoverlapped with each other on the light-receiving portion 46 h, wherebythe light-receiving portions 46 g and 46 h are prevented from becominglarge. Therefore, the photodetector 46 can be prescribed to be smallerthan that of the photodetector 45 described with reference to FIG. 22,and the photodetector 46 is less expensive than the photodetector 45.The areas occupied by the light-receiving portions 46 e to 46 h aresmaller than those occupied by the light-receiving portions 45 e to 45j. Accordingly, the beams 76 a to 76 h reflected from the informationrecording surface 41 b are unlikely to be incident upon thelight-receiving portions 46 e to 46 h, a fluctuation in a TE signal canbe reduced further than the case of using the optical pick-up headapparatus in Embodiment 10, and tracking control can be performed morestably. This also applies to the beam reflected from the surface of theoptical recording medium 41.

In order to receive the beams 75 d and 75 e by one light-receivingportion 46 g, and receive the beams 75 f and 75 g by one light-receivingportion 46 h, the pitches of patterns formed in the regions 64 d and 64f of the beam splitter 64 shown in FIG. 21 and the spatial frequencyaxis are changed slightly. The patterns formed in the regions 64 d and64 f are simple gratings in a straight line shape with an equal pitch.The light-receiving portions 46 a to 46 d are the same as those of thelight-receiving portions 45 a to 45 d.

Furthermore, the adders 802 and 803 are not required, so that an opticalinformation apparatus can be less expensive accordingly. Even if thelight-receiving portions 64 g to 46 h are placed so that the beams 76 ato 76 h are not optically incident upon the light-receiving portions 64g to 46 h, unintended stray light may be incident upon thelight-receiving portions 46 g to 46 h. However, the areas of thelight-receiving portions 46 g to 46 h are smaller than those of thelight-receiving portions 45 e to 45 j, so that the light amount ofunintended stray light is decreased relatively, and more stable trackingcontrol can be performed.

Embodiment 12

FIG. 25 schematically shows an exemplary relationship between aphotodetector 47 constituting an optical pick-up head and the beams 75 ato 75 h and 76 a to 76 h, as an example of another optical informationapparatus according to the present invention. By using the photodetector47 in place of the photodetector 45 in the optical pick-up head 402 inEmbodiment 10 and slightly changing the patterns to be formed in theregions 64 a to 64 f of the beam splitter 64, an optical pick-up head inEmbodiment 12 can be configured.

In the optical pick-up head in the present embodiment, similarly to theoptical pick-up head in Embodiment 11, the beams 75 d and 75 e arereceived by one light-receiving portion 47 g, and the beams 75 f and 75g are received by one light-receiving portion 47 h. Furthermore, thebeams 75 d and 75 e are overlapped with each other on thelight-receiving portion 47 g, and the beams 75 f and 75 g are overlappedwith each other on the light-receiving portion 47 h.

Furthermore, the patterns formed in the regions 64 a to 64 f of the beamsplitter 64 shown in FIG. 21 are provided with a power so that theastigmatism provided to the beams 75 b to 75 g when they pass throughthe cylindrical lens 57 is cancelled and these beams are focused on thephotodetector 47. Thus, the beams 75 b to 75 h on the photodetector 47are smaller than the beams 75 b to 75 h on the photodetector 46, and thelight-receiving portions 47 e to 47 h can be decreased in size comparedwith the light-receiving portions 46 e to 46 h accordingly. As a result,the photodetector 47 becomes less expensive than the photodetector 46.

Furthermore, the areas occupied by the light-receiving portions 47 e to47 h are smaller than those occupied by the light-receiving portions 46e to 46 j. Therefore, the beams 76 a to 76 h reflected from theinformation recording surface 41 b are unlikely to be incident upon thelight-receiving portions 47 e to 47 h accordingly, and more stabletracking control can be performed. This also applies to the beamreflected from the surface of the optical recording medium 41. Thelight-receiving portions 47 a to 47 d are the same as those of thelight-receiving portions 46 a to 46 d. Furthermore, even if thelight-receiving portions 47 e to 47 h are placed so that the beams 76 ato 76 h are not optically incident upon the light-receiving portions 47e to 47 h, unintended stray light may be incident upon thelight-receiving portions 47 e to 47 h. However, the light amount ofunintended stray light is decreased, relative to the decreased areas ofthe light-receiving portions 47 e to 47 h, a fluctuation in a TE signalcan be reduced further than the case of using the optical pick-up headapparatus in Embodiment 11, and more stable tracking control can beperformed.

Embodiment 13

FIG. 26 schematically shows a beam splitter 65 constituting an opticalpick-up head, as an example of another optical information apparatus ofthe present invention. By using the beam splitter 65 in place of thebeam splitter 64 in the optical pick-up head 402 shown in FIG. 21 inEmbodiment 10, an optical pick-up head in Embodiment 13 can beconfigured.

Regions 65 a to 65 g of the beam splitter 65 correspond to the regions64 a to 64 g of the beam splitter 64, and generate 1st order diffractedlight. The beam splitter 65 is different from the beam splitter 64 inthat a width h5 of the region 65 g corresponding to the region 64 g issmaller, and the regions 65 c to 65 f are larger accordingly.

By setting the regions 65 c to 65 f to be larger than the regions 64 cto 64 f shown in FIG. 21, the light amount of the beams 75 d to 75 greceived by the light-receiving portions 45 g to 45 j shown in FIG. 22is increased, and the amplification degree of the variable gainamplifier 805 can be decreased accordingly. Herein, by setting a widthh5 to be 0.35 D, the amplification degree of the variable gain amplifier805 can be decreased to about 2.3 times. Since the amplification degreeof the variable gain amplifier 805 can be decreased, in a signal inputto the variable gain amplifier 805, the electrical influence of anoffset that may be added to a signal generated and output from theadders 802, 803, etc. can be decreased.

Furthermore, even if the light-receiving portions 45 g to 45 j areplaced so that the beams 76 a to 76 g are not optically incident uponthe light-receiving portions 45 g to 45 j, unintended stray light may beincident upon the light-receiving portions 45 g to 45 j. However, whenthe light amount of the beams 75 d to 75 g received by thelight-receiving portions 45 g to 45 j is increased, the light amount ofunintended stray light is decreased relatively, so that more stabletracking control can be performed.

Furthermore, even in the case of using the beam splitter in the presentembodiment, a fluctuation in a TE signal is reduced, which occurs whenthere is a variation in a position, a width, and a depth of a grooveformed on an optical recording medium and when information is recordedin a track. Furthermore, unnecessary light is prevented from beingincident upon a light-receiving portion used for detecting a TE signalto fluctuate the TE signal, in the case where an optical recordingmedium has a plurality of information recording surfaces.

Embodiment 14

FIG. 27 schematically shows a beam splitter 66 constituting an opticalpick-up head, as an example of another optical information apparatus ofthe present invention. By using the beam splitter 66 in place of thebeam splitter 64 in the optical pick-up head 402 in Embodiment 10, anoptical pick-up head in Embodiment 14 can be configured.

Regions 66 a to 66 g of the beam splitter 66 correspond to the regions64 a to 64 g of the beam splitter 64 described with reference to FIG.21, and generate 1st order diffracted light. The beam splitter 66 isdifferent from the beam splitter 64 in that a width of the region 66 gcorresponding to the region 64 g is larger, and the regions 66 a and 66b are smaller accordingly.

By setting the regions 66 a and 66 b to be smaller than the regions 64 aand 64 h, the light amount of the beams 75 b and 75 c received by thelight-receiving portions 45 e to 45 b is decreased, and theamplification degree of the variable gain amplifier 805 described withreference to FIG. 23 can be decreased accordingly. Herein, by settingthe width h2 to be 0.35 D and the width h5 to be 0.65 D, theamplification degree of the variable gain amplifier 805 can be decreasedto about 1.4 times. Since the amplification degree of the variable gainamplifier 805 can be decreased, in a signal input to the variable gainamplifier 805, the electrical influence of an offset that may be addedto a signal generated and output from the adders 802, 803, etc. can bereduced.

Furthermore, even in the case of using the beam splitter in the presentembodiment, a fluctuation in a TE signal is reduced, which occurs whenthere is a variation in a position, a width, and a depth of a grooveformed on an optical recording medium and when information is recordedin a track. Furthermore, unnecessary light is prevented from beingincident upon a light-receiving portion used for detecting a TE signalto fluctuate the TE signal, in the case where an optical recordingmedium has a plurality of information recording surfaces.

Furthermore, by setting the width h2 to be 0.3 D to 0.5 D, thefluctuation in a TE signal amplitude can be minimized, which occurs wheninformation is recorded in some tracks on'the information recordingsurface of the optical recording medium 41 while information is notrecorded in the other tracks, and an optical information apparatuscapable of recording information stably can be provided.

Embodiment 15

FIG. 28 schematically shows a beam splitter 67 constituting an opticalpick-up head, as an example of another optical information apparatus ofthe present invention. By using the beam splitter 67 in place of thebeam splitter 64 in the optical pick-up head 402 in Embodiment 10described with reference to FIG. 21, the optical pick-up head apparatusin Embodiment 15 can be configured.

Regions 67 a to 67 g of the beam splitter 67 correspond to the regions64 a to 64 g of the beam splitter 64, and generate 1st order diffractedlight. The beam splitter 67 is different from the beam splitter 64 inthat the regions 67 h to 67 k are provided in a part of the regions 67 aand 67 b, whereby the regions 67 a and 67 b are set to be smalleraccordingly.

In the regions 67 h to 67 k, the same pattern as that of the region 67 gis recorded. More specifically, the beams generated from the regions 67h to 67 k are prevented from being received by the light-receivingportions 45 e to 45 j. By setting the regions 67 a and 67 b to besmaller, the beams 76 b and 76 c generated from the regions 67 a and 67b become unlikely to be incident upon the light-receiving portions 45 eto 45 j. In particular, this is effective when the beam generated fromthe beam splitter 67 passes through the cylindrical lens 56 describedwith reference to FIG. 20.

Furthermore, by providing the regions 67 h to 67 k, the fluctuation ofan offset in accordance with track-following contained in the beams 76 band 75 c generated from the regions 67 a and 67 b is reduced.Accordingly, the amplification degree of the variable gain amplifier 805shown in FIG. 23 can be decreased. Since the amplification degree of thevariable gain amplifier 805 can be decreased, in a signal input to thevariable gain amplifier 805, the electrical influence of an offset thatmay be added to a signal generated and output from the adders 802, 803,etc. can be decreased. Furthermore, even if the regions 67 h to 67 k areprovided, the amplitude of a TE signal is not decreased.

Furthermore, even in the case of using the beam splitter in the presentembodiment, a fluctuation in a TE signal is reduced, which occurs whenthere is a variation in a position, a width, and a depth of a grooveformed on an optical recording medium and when information is recordedin a track. Furthermore, unnecessary light is prevented from beingincident upon a light-receiving portion used for detecting a TE signalto fluctuate the TE signal, in the case where an optical recordingmedium has a plurality of information recording surfaces.

Embodiment 16

FIG. 29 schematically shows a beam splitter 68 constituting an opticalpick-up head, as an example of another optical information apparatus ofthe present invention. By using the beam splitter 68 in place of thebeam splitter 67 in Embodiment 15, an optical pick-up head in Embodiment16 can be configured.

Regions 68 a to 68 k of the beam splitter 68 correspond to the regions67 a to 67 k of the beam splitter 67, and generate 1st order diffractedlight. The beam splitter 67 is different from the beam splitter 68 inthat the patterns formed in the regions 68 h to 68 k are different fromthose formed in the regions 67 h to 67 k.

In the region 68 h, the same pattern as that of the region 68 c isrecorded. In the region 681, the same pattern as that of the region 68 dis recorded. In the region 68 j, the same pattern as that of the region68 e is recorded. In the region 68 k, the same pattern as that of theregion 68 f is recorded. Therefore, the light amount of the beams 75 dto 75 g is increased, and the amplification degree of the variable gainamplifier 805 shown in FIG. 23 can be decreased. Since the amplificationdegree of the variable gain amplifier 805 can be decreased, in a signalinput to the variable gain amplifier 805, the electrical influence of anoffset that may be added to a signal generated and output from theadders 802, 803, etc. can be decreased. Furthermore, since the lightamount of the beams 75 d to 75 g received by the light-receivingportions 45 g to 45 j is increased, the light amount of unintended straylight is decreased relatively, and more stable tracking control can beperformed.

Furthermore, even in the case of using the beam splitter in the presentembodiment, a fluctuation in a TE signal is reduced, which occurs whenthere is a variation in a position, a width, and a depth of a grooveformed on an optical recording medium and when information is recordedin a track. Furthermore, unnecessary light is prevented from beingincident upon a light-receiving portion used for detecting a TE signalto fluctuate the TE signal, in the case where an optical recordingmedium has a plurality of information recording surfaces.

The regions 64 a, 64 b, 65 a, 65 b, 66 a, 66 b, 67 a, 67 b, 68 a, and 68b in the beam splitters 64 to 68 are used for detecting a TE signalaccording to the push-pull method. When the width h2 is in a range of(1−λ/2/tp/NA)2)½·D to (1−(λ/2/tp/NA)2)½·D/2, a satisfactory TE signalcan be obtained. Furthermore, if the width h1 is equal to or less than(λ/tp/NA−1−Δ)·D, even if the beam 70 is moved on the beam splitter inaccordance with tracking control, a TE signal is not degraded at all.When the width h1 is equal to or less than 1.5·(λ/tp/NA−1-Δ)·D, asatisfactory TE signal can be obtained without any practical problem.

Herein, Δ refers to a distance at which the beam 70 is moved on the beamsplitter when the diameter of the beam 70 is standardized to be 1 on thebeam splitter. Needless to say, in the case where it is desired tofurther increase the width h2, and always keep the amplitude of a TEsignal constant, an amplitude control portion for keeping the amplitudeof a TE signal constant in accordance with tracking control may beprovided. The state of tracking control is known easily from the outputfrom the differential arithmetic operation portion 804 shown in FIG. 23.

Embodiment 17

FIG. 30 shows a configuration of a signal processing portion forgenerating a TE signal, as an example of another optical informationapparatus of the present invention. By using this signal processingportion in place of the signal processing portion in Embodiment 10described with reference to FIG. 23, an optical information apparatus ofEmbodiment 17 can be configured.

Signals I45 e to I45 j output from the light-receiving portions 45 e to45 j are subjected to a differential arithmetic operation in thedifferential arithmetic operation portion 806 in the same way as in thesignal processing portion in Embodiment 10 described with reference toFIG. 23. The signals 145 a to 145 d output from the light-receivingportions 45 a to 45 d are added to each other in the adder 807.

FIG. 31 shows a state (so-called eye patterns) when the signal obtainedfrom the adder 807 is observed with an oscilloscope. The signal outputfrom the adder 807 is input to a low pass filter 809 and an amplitudedetector 811. The low pass filter 809 outputs a signal in accordancewith the frequency component sufficiently lower than that of a signalcomposed of a mark and a space recorded on the information recordingsurfaces 41 b and 41 c of the optical recording medium 41. In the casewhere information is recorded on the information recording surfaces 41 band 41 c, a signal intensity Idc shown in FIG. 31 is output. In the casewhere information is not recorded on the information recording surfaces41 b and 41 c, a signal intensity It shown in FIG. 31 is output.

On the other hand, the amplitude detector 811 outputs a signal inaccordance with the amplitude of a signal frequency component composedof a mark and a space recorded on the information recording surfaces 41b and 41 c of the optical recording medium 41. As the amplitude detector811, a general circuit for detecting an effective value can be used.There is no particular limit to the amplitude detector 811, as long as acircuit capable of outputting a signal in accordance with an amplitude,such as a circuit for detecting an envelope, is used. In the case whereinformation is recorded on the information recording surfaces 41 b and41 c, a signal in accordance with the signal intensity Irf shown in FIG.31 is output. In the case where information is not recorded on theinformation recording surfaces 41 b and 41 c, 0 is output.

The signals output from the low pass filter 809 and the amplitudedetector 811 are input to the variable gain amplifiers 810 and 812, andamplified or attenuated to a desired signal intensity. The signalsoutput from the variable gain amplifiers 810 and 812 are added to eachother in the adder 813, and thereafter are input to the divider 808 tobe subjected to division.

The adder 807, the low pass filter 809, the amplitude detector 811, thevariable gain amplifiers 810 and 812, and the adder 813 constitute aunit for detecting a recorded portion and an unrecorded portion. Thedivider 808 constitutes an amplitude controller.

FIG. 32 shows an example of gains of the variable gain amplifiers 810and 812. A straight line k2 represents the gain of the variable gainamplifier 810, and a straight line k3 represents the gain of thevariable gain amplifier 812. The gain of the variable gain amplifier 810is constant without being dependent upon defocusing. However, the gainof the variable gain amplifier 812 is varied depending upon the state ofdefocusing of a beam condensed on the information recording surface 41 bor 41 c. Herein, the value of the gain represented by the straight linek3 is set to be 1 at a defocusing of −0.2 μm and set to be 0 at adefocusing of 0 μm. The value of the gain represented by the straightline k2 always is 1. The value of defocusing is known easily by using aFE signal.

When the gain of the variable gain amplifier 812 represented by thestraight line k3 is varied depending upon defocusing, the fluctuation ofa TE signal amplitude can be minimized, which occurs when information isrecorded in some tracks on the information recording surface of theoptical recording medium 41 while information is not recorded in theother tracks, and an optical information apparatus capable of recordinginformation stably can be provided.

The values of the gains represented by the straight lines k2 and k3 areexamples. The gains of the variable gain amplifiers 810 and 812, and theratio of a change in gain, may be set to be appropriate values inaccordance with the optical design. The optimum value of the gain may beset so that the fluctuation amount of a TE signal is minimized or sothat off-track is minimized, in accordance with the performance requiredby the optical information apparatus. Alternatively, the optimum valueof the gain may be set therebetween.

The above configuration is an example. Any configuration may be used,which can detect whether tracks on the information recording surfaces 41b and 41 c of the optical recording medium 41 are recorded orunrecorded, and can control the amplitude of a TE signal in accordancewith the recorded/unrecorded state and the state of defocusing.Furthermore, there is no limit on the number of information recordingsurfaces of the optical recording medium. The optical informationapparatus of the present invention is applicable to all the opticalrecording media, as long as an optical recording medium has aninformation recording surface in which the amplitude of a TE signal isfluctuated due to the presence of recorded tracks and unrecorded tracks.

FIG. 33 shows an example in the case where information is recorded onthe information recording surfaces 41 b and 41 c of the opticalrecording medium 41 described with reference to FIG. 20. Herein,information is recorded in the tracks Tn−2, Tn, and Tn+2, andinformation is not recorded in the tracks Tn−1 and Tn+1. Morespecifically, a recorded track and an unrecorded track are formedalternately. By scanning a beam in a direction orthogonal to the tracks,a TE signal is obtained. When recorded tracks and unrecorded tracks arepresent, the amplitude of a TE signal is fluctuated. In order tominimize this fluctuation, the gains of the variable gain amplifiers 810and 812 represented by the straight lines k2 and k3 may be adjusted. Inthe case where recorded tracks and unrecorded tracks are formedalternately, the fluctuation of a TE signal amplitude is mostconspicuous, and the gains of the variable gain amplifiers 810 and 812can be adjusted with good precision.

Embodiment 18

FIG. 34 shows an exemplary configuration of an optical pick-up head 403in the present invention, as an example of another optical informationapparatus of the present invention. The optical pick-up head 403 isdifferent from the optical pick-up head 402 in Embodiment 10 describedwith reference to FIG. 20 in that a concave lens 81 and a convex lens 82are provided between the polarized beam splitter 52 and thequarter-wavelength plate 54.

By changing the position of the concave lens 81 by an actuator 93, thespherical aberration amount given to the beam 70 can be adjusted. Thespherical aberration amount of the beam 70 condensed onto theinformation recording surfaces 41 b and 41 c is varied depending uponthe distance from the surface of the optical recording medium 41 to theinformation recording surfaces 41 b, 41 c. The spherical aberration iscorrected by using the concave lens 81 and the convex lens 82 so thatthe spherical aberration of the beam 70 condensed onto the informationrecording surfaces 41 b, 41 c is decreased. By providing the concavelens 81 and the convex lens 82, information can be recorded on both theinformation recording surfaces 41 b and 41 c with less sphericalaberration.

Herein, a diameter D1 of a beam incident upon the objective lens 56 isconstant since the opening of the objective lens 56 is limited by theaperture 55, whereas a diameter D2 of a beam incident upon the beamsplitter 64 is varied depending upon the position of the concave lens81. The size of the regions 64 a to 64 g provided in the beam splitter64 is constant. Therefore, when the diameter D2 of a beam is decreased,the light amount of the beams 75 b to 75 c generated in the regions 64 ato 64 b is increased, and the light amount of the beams 75 d to 75 ggenerated in the regions 64 c to 64 f is decreased. If the gain of thevariable gain amplifier 805 represented by the straight line k1 remainsconstant, the offset fluctuation in accordance with track-following of asignal output from the differential arithmetic operation portion 801cannot be decreased appropriately.

FIG. 35 shows a relationship between a ratio D2/D1 of the beam diametersin the present embodiment and the gain of the variable gain amplifier805 represented by the straight line k1. As the ratio D2/D1 of the beamsis decreased, the gain of the variable gain amplifier 805 is increased.

FIG. 36 shows a relationship between a driving voltage of the actuator93 shown in FIG. 34 and the ratio D2 /D1 of the beam diameters. Thedriving voltage of the actuator 93 and the ratio D2/D1 of the beamdiameters has a correlation. Herein, the gain of the variable gainamplifier 805 represented by the straight line k1 is controlled inaccordance with the driving voltage of the actuator 93. It is difficultto measure the ratio D2/D1 of the beam diameters, whereas the drivingvoltage of the actuator 93 is known easily. Even in the case where theconcave lens 81 is displaced so that the spherical aberration of thebeam condensed on the information recording surface 41 b or 41 c isdecreased, the offset fluctuation in accordance with track-following ofthe signal output from the differential arithmetic operation portion 801can be reduced appropriately. More specifically, the reliability of anoptical information apparatus for recording information on an opticalrecording medium having a plurality of information recording surfacescan be enhanced.

Embodiment 19

FIG. 37 shows a configuration of another optical pick-up head in thepresent invention. Alight source 1 such as a semiconductor laser emits alinearly polarized beam 70 with a wavelength λ1 of 405 nm. The beam 70emitted from the light source 1 is converted to parallel light by acollimator lens 53 to be incident upon a polarized beam splitter 52. Thebeam having passed through the polarized beam splitter 52 has itsoptical path bent by a rising mirror 24 and passes through aquarter-wavelength plate 54 to be converted to circularly polarizedlight. Thereafter, the beam 70 is incident upon the objective lens 56 tobe condensed on it. The objective lens 56 has a focal length f of 2 mm,and a numerical aperture NA of 0.85. The objective lens 56 is driven byan objective lens driving apparatus (not shown), and the beam 70 passesthrough a transparent protective layer with a thickness of 0.1 mm to becondensed onto a recording surface of an optical recording medium 40.The optical recording medium 40 is provided with a continuous groove tobe a track, and a pitch tp is 0.32 μm.

The beam 70 reflected from the optical recording medium 40 passesthrough the objective lens 56 and the quarter-wavelength plate 54 to beconverted to linearly polarized light whose plane of polarization isshifted by 90° from an ingoing path, and is reflected from the polarizedbeam splitter 52 as a beam splitting unit. The beam 70 reflected fromthe polarized beam splitter 52 to have its direction changed passesthrough an opening limit element 83. The beam 70 passes through adiffraction element 22 in contact with the opening limit element 83, andis split to a plurality of beams 210 and 21 a to 21 e. The split beamspass through a condensing lens 59 to be converted to convergent light.The convergent light passes through a cylindrical lens 57 to be providedwith astigmatism. Thereafter, the resultant beam 70 is incident upon aphotodetector 51. The beam 70 incident upon the photodetector 51 isoutput as an electric signal.

FIG. 38 schematically shows configurations of the diffraction element 22and the opening limit element 83. FIG. 39 schematically shows arelationship between the shape of the light-receiving portion of thephotodetector 51 and the beams 210 and 21 a to 21 d.

The diffraction element 22 has four regions 22 a to 22 e. Thediffraction element 22 transmits a large part of the incident beam 70 togenerate 0th order diffracted light 210, and diffracts a part hereof togenerate beams 70 a to 70 e from the regions 22 a to 22 e, respectively.

The photodetector 51 has 8 light-receiving portions 51 a to 51 h intotal. The light-receiving portions 51 a to 51 h are used for detectinga signal. The light-receiving portions 51 a to 51 d receive the beam210. The light-receiving portion 51 g receives the beam 21 c. Thelight-receiving portion 51 e receives the beam 21 a. The light-receivingportion 51 f receives the beam 21 c. The light-receiving portion 51 hreceives the beam 21 d. The region 22 e has characteristics so as todiffract light at a diffraction angle larger than the other regions, andthe beam diffracted in the region 22 e is not incident upon thephotodetector 51. The respective ends of the light-receiving portions 51a to 51 h of the photodetector 51 are provided with electrodes 84, andcurrent signals I51 a to I51 h in accordance with the received lightamounts are output from the electrode 84 to a semiconductor circuit (notshown).

A FE signal is obtained by an arithmetic operation (I51 a+I51 c)−(I51b+I51 d) according to the astigmatism method, and the position of theobjective lens 56 is controlled based on the FE signal.

Furthermore, a TE signal is obtained by an arithmetic operation (I51g−I51 h)−k·(51 e−I51 f). The position of the objective lens iscontrolled based on the TE signal thus obtained, whereby a signal can berecorded/reproduced.

In Embodiment 19, the diffraction element 22 is divided by straightlines. However, the diffraction element 22 may be divided by anypositional shape so that the characteristics of a TE signal areoptimized in accordance with the characteristics and the like of theoptical recording medium 40. In the above-mentioned arithmeticoperation, k is a real number and set to be an appropriate valuedepending upon the divided positions of the grating element 22 and thecharacteristics of the optical recording medium 40.

Furthermore, during reproduction of information, an information signal(hereinafter, referred to as “RF signal”) recorded on an informationrecording medium is obtained by I51 a+I51 b+I51 c+I51 d.

At this time, the beams 210, and 21 a to 21 d are incident upon thephotodetector 51. In addition, light reflected from a surface 40 d of atransparent substrate of the optical recording medium 40 returns to thephotodetector 51 as divergent light, as shown by the stray light 21 inFIG. 37.

The stray light 21 also is incident upon the condensing lens 59 afterpassing through the diffraction element 22. However, since the straylight 21 is divergent light that is diverged larger compared with thenormal beam 70, the stray light 21 may be incident upon the surface ofthe photodetector 51 under the condition of being spread larger than thespot formed by the beam 210.

The stray light 21 may be incident upon the light-receiving portions fora signal. Particularly, in the case where the stray light 21 is incidentupon the light-receiving portions 51 e to 51 h that receive thediffracted light beams 21 a to 21 d with a relatively small lightamount, signal quality is degraded greatly. Consequently, trackingcontrol becomes unstable, and information cannot be recorded/reproducedwith high reliability. In Embodiment 19, the opening limit element 83 isinserted in the path to block ambient light of the stray light 21,whereby a spot 320 by the stray light 21 on the surface of thephotodetector 51 can be decreased as shown in FIG. 39. Therefore, thestray light 21 can be prevented from being incident upon the lightreceiving portions 51 e to 51 h for receiving the diffracted light beams21 a to 21 d.

Furthermore, when the stray light 21 is minimized, although it is notincident upon the light-receiving portions directly, unnecessary lighttraveling to the light-receiving portions due to reflection from theinner surface of a lens barrel (not shown) and the inner surface of anoptical head can be blocked, whereby stable tracking control can beperformed.

The stray light 21 also is diffracted by the diffraction element 22. Inorder to prevent the diffracted light thereof from being incident uponthe light-receiving portions 51 e to 51 h, the stray light 21 isdiffracted at a large diffraction angle so as to travel to an outside ofthe photodetector 51 in the region 22 e at the center of the diffractionelement 22.

The opening diameter of the opening limit element 83 is set to be adiameter 83 a or more determined by the NA of the objective lens asshown in FIG. 38 so as not to block the normal beam 70 reflected fromthe recording surface of the optical recording medium 40.

Furthermore, when the objective lens follows a track to be displaced ina tracking direction, the position of the beam 70 also is changed. Evenin this case, in order not to block the beam 70, it is desirable thatthe opening of the opening limit element 83 has an elliptical shape witha major diameter 83 b that is increased considering the displacement ofthe objective lens in a tracking direction.

Furthermore, in order to block the stray light 21 as much as possible,the opening limit element 83 is placed at a position where the diameterof the stray light 21 is spread as largely as possible with respect tothe beam 70 (i.e., at a position where the light amount of the straylight 21 having passed through the opening for passage of the beam 70 isminimized), whereby the stray light 21 can be blocked most effectively.For this purpose, in Embodiment 19, the opening limit element 83 isplaced at a position in contact with the diffraction element 22.

The reason for the above is as follows. As shown in FIG. 37, the straylight 21 travels to the photodetector 51 while being diverged comparedwith the beam 70. Therefore, if the opening is limited at a farthestpossible position from the optical recording medium 40, much of thestray light 21 can be blocked. However, on the photodetector 51 sidefrom the diffraction element 22, the beams 21 a to 21 e split by thediffraction element 22 are spread with respect to the beam 210. If alarge opening diameter is set with respect to the beam 210 so as not toblock the beams 21 a to 21 e, the blocking amount of the stray light 21is decreased accordingly.

Furthermore, in the case where the opening center of the opening limitelement 83 is not matched with the center of the diffraction element 22,an imbalance is caused in the amounts of light that passes and is splittherebetween. Consequently, a TE signal has an abnormal error, degradingtracking performance. In Embodiment 19, the opening limit element 83 isplaced at a position in contact with the diffraction element 22.Therefore, the alignment between the opening limit element 83 and thediffraction element 22 can be performed easily. For example, it also ispossible that before the diffraction element 22 is incorporated into anoptical head apparatus, the opening limit element 83 is aligned to befixed with the diffraction element 22 while the division pattern of thediffraction element 22 is being observed, and they are attached to anoptical head apparatus. Thus, the number of steps of assembling anoptical head apparatus is decreased, and an inexpensive optical headapparatus can be provided.

In Embodiment 19, the opening limit element 83 and the diffractionelement 22 are composed of separate members. However, this is notnecessarily required. For example, as shown in FIG. 40, a portion of thediffraction element 22 corresponding to an opening limit element ispartitioned as a region 22 f having other diffraction characteristics,and this portion is provided with characteristics so as to diffract allthe light beams having passed through the region 22 f in such a mannerthat they are not incident upon the photodetector 51. Even in this case,the same effects can be obtained. Furthermore, in this case, it is notnecessary to align the opening limit element 83. The region 22 f mayhave any configuration as long as it has a function of substantiallyblocking the stray light 21 to the light-receiving portions 51 e to 51h. For example, the region 22 f may be made of a reflective film or anabsorbent film. Furthermore, the region 22 f may be a diffractiongrating having a high diffraction efficiency.

Because of the above-mentioned configuration, a satisfactory TE signalcan be obtained, which is less influenced by stray light reflected froma protective layer of an information recording medium, and an opticalhead apparatus capable of recording/reproducing information with highreliability can be provided.

Furthermore, by using the optical pick-up head in Embodiment 19 in placeof the optical pick-up head 4 in the optical information apparatus ofEmbodiment 1, an optical information apparatus can be configured inwhich a signal output with high reliability is obtained, andsatisfactory recording/reproducing characteristics are obtained.

Furthermore, the reflective surface for generating the stray light 21 isnot limited to the surface 40 d of the transparent substrate. In thecase where the optical recording medium 40 has a plurality of recordingsurfaces, stray light also is generated from the other surfaces than therecording surface where information is recorded/reproduced. The presentinvention also is effective even in this case.

Embodiment 20

In Embodiment 20, a region required for a TE signal is divided by ahologram element, and an electrically optimum correction coefficient isdetermined.

FIG. 41 shows a configuration of an optical pick-up head constituting anoptical information apparatus of Embodiment 20. Alight beam emitted froma semiconductor laser (light source) 1 is collimated by a collimatorlens 53. The collimated light beam is reflected from a beam splitter(splitting unit) 103, and condensed onto an information recordingsurface 40 b of an optical recording medium 40 by an objective lens(condensing unit) 56. On the information recording surface 40 b of theoptical recording medium 40, a track in which a mark and a space areplaced selectively, or a guide groove for placing a mark and a space isarranged as a track concentrically or spirally at a predeterminedinterval. The objective lens 56 is moved in an optical axis directionand in a direction traversing a track in accordance with the deflectionof a surface of an optical recording medium and the eccentricity ofoptical recording medium by actuators 91 and 92.

The light beam reflected/diffracted from the information recordingsurface 40 b passes through the objective lens 56 again to becollimated, passes through the beam splitter (splitting unit) 103, andis partially diffracted by a hologram element (divider) 201. The lightbeam having passed through the hologram element 201 is condensed by adetection lens 107. The detection lens 107 is a complex functional lenshaving both the functions of the condensing lens 59 and the cylindricallens 57 in Embodiment 1 described with reference to FIG. 2. Aphotodetector eight detector) 46 receives a light beam 203 anddiffracted light 204 having passed through the hologram element 201.

FIG. 42 shows a relationship between the division of the hologramelement 201 and a light beam. The hologram element 201 is divided into 6regions 220 a to 220 f by three dividing lines 201 a, 201 b, and 201 c.Alight beam 221 is substantially circular, and portions where ±1st orderlight and 0th order light diffracted by a track of an optical recordingmedium are overlapped with each other are represented by shaded parts.The shaded parts correspond to a first region mainly containing a TEsignal. From the regions 220 c and 220 d including the first region, asignal mainly containing a TE signal component can be obtained. On theother hand, the regions 220 a, 220 b, 220 e, and 220 f correspond to asecond region mainly containing an offset component of a TE signal, anda signal mainly containing an offset component can be obtained from thesecond region.

FIG. 43 shows a distribution of the diffraction efficiency of thehologram element 201 along alternate long and short dashed lines 222. Ahorizontal axis represents a position in a tangential direction (tracktangent direction), and a vertical axis represents a diffractionefficiency. Broken lines represent positions of intersection points ofthe dividing lines 201 b and 201 c. As shown in FIG. 43, a diffractionefficiency η2 outside of the dividing lines 201 b and 201 c is set to behigher than a diffraction efficiency η1 inside of the dividing lines 201b and 201 c. In this manner, the efficiency at which a light beam in theregions mainly containing an offset component reaches the photodetector46 is enhanced.

FIG. 44 shows a configuration of the photodetector 46 and an electriccircuit. 0th order light receiving portions 46 a to 46 d provided in thephotodetector 46 receive the light beam 203 that is 0th order lighthaving passed through the hologram element 201. Signals output from thelight-receiving portions 46 a to 46 d are used to detect a FE signal andan information reproducing signal. Herein, the detailed description ofthe detection of a FE signal is omitted.

The light-receiving portions 46 e to 46 h receive the diffracted light204 (FIG. 41) diffracted by the hologram element 201, and output currentsignals in accordance with the light amounts thereof. Thelight-receiving portion 46 e receives light having passed through theregion 220 c shown in FIG. 42, and the light-receiving portion 46 greceives light having passed through the region 220 d. From thelight-receiving portions 46 e and 46 g, a signal mainly containing a TEsignal component can be obtained. An IV amplifier (converter) 130converts the current signal from the light-receiving portion 46 e to avoltage signal. An IV amplifier (converter) 131 converts the currentsignal from the light-receiving portion 46 g to a voltage signal.

Furthermore, the light-receiving portion 46 f receives light havingpassed through the regions 220 a and 220 e shown in FIG. 42, and thelight-receiving portion 46 h receives light having passed through theregions 226 b and 220 f. From the light-receiving portions 46 f and 46h, a signal mainly containing an offset component can be obtained. An IVamplifier (converter) 132 converts the current signal from thelight-receiving portion 46 h to a voltage signal. An IV amplifier(converter) 133 converts the current signal from the light-receivingportion 46 f to a voltage signal.

A differential arithmetic operation unit 134 receives output signalsfrom the IV amplifiers 130 and 131 to output a differential signalthereof. This differential signal mainly contains a TE signal component.On the other hand, the differential arithmetic operation portion 135receives outputs from the IV amplifiers 132 and 133 to output adifferential signal thereof. This differential signal mainly contains anoffset component. The signal output from the differential arithmeticoperation portion 135 is multiplied by a gain (coefficient) k by avariable gain amplifier 136, whereby a signal multiplied by k is output.The differential arithmetic operation portion (TE signal generator) 137receives output signals from the differential arithmetic operationportion 134 and the variable gain amplifier 136 to output a differentialsignal thereof.

The gain k of the variable gain amplifier 136 is determined so that thefluctuation amount of a DC component of a signal output from thedifferential arithmetic operation portion 135 is equal to thefluctuation amount of a DC component of a signal output from thevariable gain amplifier 136 when an objective lens is moved. From thedifferential arithmetic operation portion 137, a TE signal that is notsubjected to offset fluctuation is obtained, even when the objectivelens is moved.

The gain k depends upon the ratio between the interval between thedividing lines 201 b and 201 c and the diameter of the light beam 221,and the light intensity distribution in the light beam 221. Herein, bysetting the diffraction efficiency η2 of the regions 220 a, 220 b, 220e, and 220 f to be twice the diffraction efficiency η1 of the regions220 c and 220 d, the gain k can be set to be about 1.

Assuming that the electric offset occurring in each of the IV amplifiers130 to 133 is ΔE on average, the gain k in the conventional example isabout 2, so that an electric offset that is 6 times ΔE occurs in acorrected TE signal in the worst case. However, in Embodiment 20, sincethe gain k may be about 1, an electric offset may be 4 times ΔE in theworst case. Thus, the amount of an offset that is varied depending uponthe temperature and the like can be reduced to ⅔ of the conventionalexample.

In this example, the gain that is a coefficient for reducing an offsetof a TE signal can be set to be an optimum value on a head or opticalrecording medium basis. Therefore, an offset of a TE signal can beminimized. Furthermore, the positions of the dividing lines of thehologram element are determined independently from a diffractionefficiency, so that the degree of freedom can be ensured at which anoptimum shape can be used as a dividing pattern.

If the diffraction efficiency in the region for obtaining a signalmainly containing an offset component is enhanced, the gain k can bedecreased further. Therefore, the amount of an electric offset that isvaried depending upon the temperature and the like can be decreased.

Furthermore, in such an example, the transmission efficiency of 0thorder light for obtaining a RF signal in a tangential direction also isvaried. However, the influence on a RF signal can be reduced by awaveform equalization, a maximum likelihood decoding method (PRML), andthe like.

As an example using another divider, FIG. 45 shows a relationshipbetween the division of a hologram element (divider) 241 and a lightbeam in another example. The hologram element 241 is used in place ofthe hologram element 201 described with reference to FIG. 42. Thehologram element 241 is divided into 6 regions 245 a to 245 f bydividing lines 241 a, 241 b, and 241 c. The diffracted light generatedfrom each divided region is incident upon a detection system fordetection in a similar manner to that described above with reference toFIG. 44.

FIG. 46 shows a diffraction efficiency distribution along alternate longand short dashed lines 246. A horizontal axis represents a position in atangential direction (direction traversing a track), and a vertical axisrepresents a diffraction efficiency. The diffraction efficiency ischanged linearly so as to be η3 at a center portion and η4 at both ends.Broken lines present positions of intersection points of the alternatelong and short dashed lines 246 and the dividing lines 241 b and 241 c.

Even with the above-mentioned configuration, the efficiency at whichlight having passed through the regions 245 a, 245 b, 245 e, and 245 fmainly containing an offset reaches the photodetector 46 (FIG. 41) ishigh, so that the gain k of the variable gain amplifier can bedecreased. Therefore, the amount of an offset due to the fluctuation ofan electric offset that is varied depending upon the temperature and thelike can be reduced.

As an example using another divider, FIG. 47 shows a relationshipbetween the division of a hologram element (divider) 251 and a lightbeam in still another example. The hologram element 251 is used in placeof the hologram element 201 described with reference to FIG. 42. Thehologram element 251 is divided into 6 regions 255 a to 255 f by thedividing lines 251 a, 251 b, and 251 c. Diffracted light generated fromeach divided region is incident upon a detection system for detection ina similar manner to that of the example described with reference to FIG.44.

FIG. 48 shows a diffraction efficiency distribution along alternate longand short dashed lines 256. A horizontal axis represents a position in aradial direction (direction traversing a track), and a vertical axisrepresents a diffraction efficiency. The hologram element 251 isproduced so that the diffraction efficiency is 5 at a center, and is 6,higher than η5, at both ends. Broken lines represent intersection pointsof dotted lines 257, 258 and the alternate long and short dashed lines256.

Even with such a configuration, among the regions 255 a, 255 b, 255 e,and 255 f mainly containing an offset, in the circumferential portion ofa light beam where the area is changed largely due to the movement of anobjective lens, the efficiency at which a light beam reaches thephotodetector 46 (FIG. 41) is high, so that the gain k of the variablegain amplifier can be decreased. Therefore, the amount of an offset dueto the fluctuation of an electric offset that is varied depending uponthe temperature and the like can be reduced. Thus, the diffractionefficiency may be varied in the same region. In such an example, thediffraction efficiency in a tangential direction is changed less, sothat the partial difference in a transmission efficiency of 0th orderlight for obtaining a RF signal can be decreased to reduce the influenceon a RF signal.

As an example using another divider, FIG. 49 shows a relationshipbetween the division of a hologram element (divider) 261 in stillanother example and a light beam. The hologram element 261 is used inplace of the hologram element 201 described with reference to FIG. 42.The hologram element 261 is divided into 6 regions 265 a to 265 f bydividing lines 261 a, 261 b, and 261 c. The diffracted light generatedfrom each divided region is incident upon a detection system fordetection in a similar manner to that of the example described withreference to FIG. 44.

FIG. 50 shows a diffraction efficiency distribution along alternate longand short dashed lines 266. A horizontal axis represents a position in aradial direction, and a vertical axis represents a diffractionefficiency. The hologram element 261 is produced so that the diffractionefficiency is η7 at a center, and is η8 higher than η7 at both ends.Broken lines represent intersection points of dotted lines 267, 268 andthe alternate long and short dashed lines 266.

Even with such a configuration, among the regions 265 a, 265 b, 265 e,and 265 f mainly containing an offset, in the circumferential portion ofa light beam where the area is changed largely due to the movement of anobjective lens, the efficiency at which a light beam reaches thephotodetector 46 (FIG. 41) is high, so that the gain k of the variablegain amplifier can be decreased. Therefore, the amount of an offset dueto the fluctuation of an electric offset that is varied depending uponthe temperature and the like can be reduced. The diffraction efficiencymay be varied along a light beam.

Embodiment 21

In Embodiment 21, a region required for a TE signal is split and dividedby a hologram element, and these divided regions are displaced opticallyfor an arithmetic operation.

The configuration of an optical system is substantially the same as thatof Embodiment 20, so that the diagram of the configuration is omitted.Embodiment 21 is different from Embodiment 20 in that a hologram element(divider) 301 is used in place of the hologram element 201 describedwith reference to FIG. 42, and a photodetector (light detector) 303 isused in place of the photodetector 46.

FIG. 51 shows a relationship between the division of the hologramelement (divider) 301 and a light beam. The hologram element 301 isdivided into 6 regions 302 a to 302 f by 3 dividing lines 301 a, 301 b,and 301 c. A light beam 321 is substantially circular, and portionswhere ±1st order light and 0th order light diffracted by a track of anoptical recording medium are overlapped with each other are representedby shaded parts. The shaded parts correspond to regions mainlycontaining a TE signal. From the regions 302 c and 302 d, a signalmainly containing a TE signal component can be obtained. On the otherhand, from the regions 302 a, 302 b, 302 e, and 302 f, a signal mainlycontaining an offset component of a TE signal can be obtained.

FIG. 52 shows a distribution of a diffraction efficiency along alternatelong and short dashed lines 322. A horizontal axis represents a positionin a tangential direction (track tangent direction), and a vertical axisrepresents a diffraction efficiency. Broken lines represent positions ofintersection points of the dividing lines 301 b, 301 c and the alternatelong and short dashed lines 322. As shown in FIG. 52, a diffractionefficiency η10 outside of the dividing lines 301 b and 301 c is set tobe about twice a diffraction efficiency η9 inside of the dividing lines301 b and 301 c. In this manner, the efficiency at which a light beam inthe regions mainly containing an offset component reaches thephotodetector is enhanced.

FIG. 53 shows a configuration of the photodetector 303 and an electriccircuit. 0th order light receiving portions 303 a to 303 d provided inthe photodetector 303 receive a light beam 331 that is 0th order lighthaving passed through the hologram element 301. Signals output from thelight-receiving portions 303 a to 303 d are used to detect a FE signaland an information reproducing signal. The light-receiving portions 303e and 303 f receive light diffracted by the hologram element 301, andoutput current signals in accordance with the light amounts thereof. Thelight-receiving portion 303 e receives light having passed through theregions 302 b, 302 c, and 302 f, and the light-receiving portion 303 freceives light having passed through the regions 302 a, 302 d, and 302e.

An IV amplifier (converter) 340 converts the current signal from thelight-receiving portion 303 e to a voltage signal. An IV amplifier(converter) 341 converts the current signal from the light-receivingportion 303 f to a voltage signal. From the regions 302 a and 302 b, asignal mainly containing a TE signal component can be obtained, and fromthe regions 302 a, 302 b, 302 e, and 302 f, a signal mainly containingan offset component can be obtained. Thus, each light-receiving portionis placed so that regions on different sides with respect to thedividing line 301 a belong to the same light-receiving portion. This canreduce an offset due to the movement of an objective lens. Adifferential arithmetic operation portion 342 receives output signalsfrom the IV amplifiers 340 and 341, and outputs a differential signalthereof. Because of this, even if an objective lens is moved, a TEsignal without the fluctuation in an offset is obtained.

In Embodiment 21, two IV amplifiers may be used. Therefore, assumingthat an electric offset occurring in each IV amplifier is ΔE on average,an electric offset that is twice ΔE occurs in a corrected TE signal inthe worst case. Thus, the amount of an offset that is varied dependingupon the temperature and the like can be reduced to ⅓ of theconventional example.

Embodiment 22

In Embodiment 22, a far field is divided by a prism, and is electricallymultiplied by a correction coefficient to reduce a TE signal offset.

FIG. 54 shows a configuration of an optical pick-up head constituting anoptical information apparatus of Embodiment 22. Alight beam emitted froma semiconductor laser (light source) 1 is collimated by a collimatorlens 53, reflected from a beam splitter (splitting unit) 103, andcondensed onto an information recording surface 40 b of an opticalrecording medium (optical recording medium) 40 by an objective lens(condensing unit) 56. The objective lens 56 is moved in an optical axisdirection and in a direction traversing a track in accordance with thedeflection of a surface of an optical recording medium and theeccentricity of an optical recording medium by actuators 91 and 92. Thelight beam reflected/diffracted from the information recording surface40 b passes through the objective lens 56 again to be collimated, andpasses through the beam splitter (splitting unit) 103. A part of thelight is reflected from a beam splitter (divider) 104, and a partthereof passes therethrough.

The light having passed through the beam splitter 104 is condensed by adetection lens 107, and received by a photodetector (light detector) 30.On the other hand, the light reflected from the beam splitter 104 issplit by a prism (splitting unit) 105. The split light is condensed by adetection lens 106 to be detected by a photodetector (light detector)305.

FIG. 55 shows a relationship between the division of the prism 105 and alight beam. The prism 105 is divided into 6 regions 420 a to 420 f bythree dividing lines 410, 411, and 412. Alight beam 421 is substantiallycircular, and portions where 1st order light and 0th order lightdiffracted by a track of an optical recording medium are overlapped witheach other are represented by shaded parts. The shaded parts correspondto regions mainly containing a TE signal. From the regions 420 c and 420d, a signal mainly containing a TE signal component can be obtained. Onthe other hand, from the regions 420 a, 420 b, 420 e, and 4201, a signalmainly containing an offset component of a TE signal can be obtained.

FIG. 56 shows a distribution of a transmittance along alternate long andshort dashed lines 422. A horizontal axis represents a position in atangential direction (track tangent direction), and a vertical axisrepresents a transmittance. Broken lines represent positions of thedividing lines 411 and 412. In this manner, a transmittance η12 outsideof the dividing lines 411 and 412 is set to be higher than atransmittance η11 inside of the dividing lines 411 and 412. Thus, theefficiency at which a light beam in the regions mainly containing anoffset component reaches the photodetector 305 is enhanced.

FIG. 57 shows a configuration of the photodetector 305 and an electriccircuit. Six light-receiving portions 305 a to 305 f receive lightreflected from the beam splitter 104 and split by the prism 105, andoutput current signals in accordance with the light amounts thereof.Alight-receiving portion 305 c receives light having passed through theregion 420 c shown in FIG. 55, and a light-receiving portion 305 dreceives light having passed through the region 420 d. From thelight-receiving portions 305 c and 305 d, a signal mainly containing aTE signal component can be obtained. An IV amplifier (converter) 130converts the current signal from the light-receiving portion 305 c to avoltage signal. An IV amplifier (converter) 131 converts the currentsignal from the light-receiving portion 305 d to a voltage signal.

Furthermore, the light-receiving portion 305 a receives light havingpassed through the region 420 a. The light-receiving portion 305 ereceives light having passed through the region 420 e. Thelight-receiving portion 305 b receives light having passed through theregion 420 b. The light-receiving portion 305 f receives light havingpassed through the region 420 f. From the light-receiving portions 305a, 305 b, 305 e, and 305 f, a signal mainly containing an offsetcomponent can be obtained. An IV amplifier (converter) 132 converts thecurrent signals from the light-receiving portions 305 b and 305 f tovoltage signals. An IV amplifier (converter) 133 converts the currentsignals from the light-receiving portions 305 a and 305 e to a voltagesignal.

A differential arithmetic operation unit 134 receives output signalsfrom the IV amplifiers 130 and 131 to output a differential signalthereof. This differential signal mainly contains a TE signal component.On the other hand, the differential arithmetic operation portion 135receives outputs from the IV amplifiers 132 and 133 to output adifferential signal thereof. This differential signal mainly contains anoffset component. The signal output from the differential arithmeticoperation portion 135 is multiplied by a gain k by a variable gainamplifier 136, whereby a signal multiplied by k is output. Thedifferential arithmetic operation portion 137 receives output signalsfrom the differential arithmetic operation portion 134 and the variablegain amplifier 136 to output a differential signal thereof.

The gain k of the variable gain amplifier 136 is determined so that thefluctuation amount of a DC component of a signal output from thedifferential arithmetic operation portion 135 is equal to thefluctuation amount of a DC component of a signal output from thevariable gain amplifier 136 when an objective lens is moved. From thedifferential arithmetic operation portion 137, a TE signal that is notsubjected to offset fluctuation is obtained, even when the objectivelens is moved.

The gain k depends upon the ratio between the interval from the dividinglines 411 to 412 and the diameter of the light beam 421, and the lightintensity distribution in the light beam 421. Herein, by setting thetransmittance η12 of the regions 420 a, 420 b; 420 e, and 420 f to betwice the transmittance η11 of the regions 420 c and 420 d, the gain kcan be set to be about 1.

Assuming that the electric offset occurring in each of the IV amplifiers130 to 133 is ΔE on average, the gain k may be about 1 in Embodiment 22in the same way as in Embodiment 20. Therefore, an electric offset thatis 4 times ΔE occurs in the worst case. Thus, the amount of an offsetthat is varied depending upon the temperature and the like can bereduced to ⅔ of the conventional example.

In the example in Embodiment 22, the gain that is a coefficient forreducing an offset of a TE signal can be set to be an optimum value on ahead or optical recording medium basis in the same way as in Embodiment20. Therefore, an offset of a TE signal can be minimized. Furthermore,the positions of the dividing lines of the hologram element aredetermined independently from a diffraction efficiency, so that thedegree of freedom can be ensured at which an optimum shape can be usedas a dividing pattern. Furthermore, compared with the case of utilizinga hologram element, a loss caused by diffraction is small because of theuse of a prism, and light use efficiency can be enhanced; therefore, theinfluence of an electric offset can be reduced.

Embodiment 23

In Embodiment 23, a part of light is diffracted by a hologram elementthat is moved integrally with an objective lens, and regions arereplaced.

FIG. 58 shows a configuration of an optical pick-up head constituting anoptical information apparatus of Embodiment 23. A linearly polarizedlight beam emitted from a semiconductor laser eight source) 1 iscollimated by a collimator lens 53, is reflected from a beam splitter(splitting unit) 103, passes trough a polarized light hologram element(divider) 501 and a quarter-wavelength plate 54 to be circularlypolarized light, and is condensed onto an information recording surface40 b of an optical recording medium 40 by an objective lens (condensingunit) 56. The objective lens 56, the polarized light hologram element501, and the quarter-wavelength plate 54 are moved in an optical axisdirection and in a direction traversing a track in accordance with thedeflection of a surface of an optical recording medium and theeccentricity of an optical recording medium by actuators 91 and 92. Thelight beam reflected/diffracted from the information recording surface40 b passes through the objective lens 56 again to be collimated, andpasses through the quarter-wavelength plate 54 to be linearly polarizedlight whose plane of polarization is shifted by 90° from that of aningoing light beam.

The linearly polarized light is partially diffracted by the polarizedlight hologram element 501, and has its traveling direction changed. Thelight output from the polarized light hologram element 501 passesthrough a beam splitter (splitting unit) 103, is provided withastigmatism by the detection lens 107, is condensed, and is received bya photodetector (light detector) 30.

FIG. 59 shows a relationship between the division of the polarized lighthologram element 501 and a light beam. The polarized light hologramelement 501 is divided into 6 regions by 4 dividing lines 510, 511, 512,and 513. Among them, the regions 520 a and 520 b correspond to a firstregion mainly containing a TE signal component. The first region has nogrooves of the hologram, so that light beams pass therethroughcompletely. The regions 521 a, 521 b, 521 c and 521 d correspond to asecond region mainly containing an offset component. The second regionhas blazed grooves, so that a light beam is diffracted in a particulardirection in the second region. The second region is divided into fourregions by the dividing line 511 substantially parallel to a tracktangent direction and the dividing line 513 substantially parallel to adirection orthogonal to a track.

FIG. 60 shows configurations of the photodetector 30 and an electriccircuit. The photodetector 30 is composed of four light-receivingportions 30 a to 30 d divided by a dividing line 530 substantiallyparallel to a track tangent direction and a dividing line 531substantially parallel to a direction orthogonal to a track. Lighthaving passed through the region 520 a of the polarized hologram element501 shown in FIG. 59 becomes a light beam 540 a that extends across thelight-receiving portions 30 a and 30 b. Light having passed through theregion 520 b becomes a light beam 540 b that extends across thelight-receiving portions 30 c and 30 d. Thus, the light beam havingpassed through the regions 520 a and 520 b mainly containing a TE signalcomponent is split into 4 regions by the dividing lines 510, 512substantially parallel to a track tangent direction on the hologramelement 501 and the dividing line 531 substantially parallel to adirection orthogonal to a track.

On the other hand, light having passed through four regions sandwichedby the dividing lines 510 and 512 shown in FIG. 59 is placed at aposition diagonally opposed to the position of the light having passedthrough the first region. More specifically, the light having passedthrough the region 521 a becomes a light beam 541 d and is received bythe light-receiving portion 30 c, and the light having passed throughthe region 521 b becomes a light beam 541 c and is received by thelight-receiving portion 30 b. The light having passed through the region521 c becomes a light beam 541 b and is received by the light-receivingportion 30 d. The light having passed through the region 521 d becomes alight beam 541 a and is received by the light-receiving portion 30 a.

The light received by the light-receiving portion 30 a is output as acurrent signal, and converted to a voltage signal by the IV amplifier130. The signal output from the IV amplifier 130 is defined as a signalA. The light received by the light-receiving portion 30 d is output as acurrent signal, and converted to a voltage signal by the IV amplifier131. The signal output from the IV amplifier 131 is defined as a signalB. The light received by the light-receiving portion 30 b is convertedto a voltage signal by the IV amplifier 133. The signal output from theIV amplifier 133 is defined as a signal C. The light received by thelight-receiving portion 30 c is converted to a voltage signal by the IVamplifier 132. The signal output from the IV amplifier 132 is defined asa signal D.

An adder 550 receives the signals A and C, and outputs a sum (A+C). Anadder 551 receives the signals B and D, and outputs a sum (B+D). Adifferential arithmetic operation portion 552 receives signals from theadders 550 and 551, and outputs a differential signal {(A+C)−(B+D)}. ATEsignal can be obtained from the signal of the differential arithmeticoperation portion 522.

In this example, a FE signal is detected by the astigmatism method. Alight beam is provided with astigmatism by the detection lens 107 shownin FIG. 58. Therefore, when the distance between the objective lens 56and the optical recording medium 40 is changed, a spot on thephotodetector 30 is distorted, whereby the light beam is changed from asubstantially circular shape to an elliptical shape to become a focalline. By setting the direction for providing astigmatism so that anangle formed by the focal line and the dividing line of thephotodetector 30 is 45°, and generating a signal (A+D)−(B+C), a focuserror can be detected.

Furthermore, when using an optical recording medium dedicated toreproducing, on which information is recorded in a pit string, the phaseof the signal (A+D) is compared with that of the signal (B+C), wherebytracking control can be performed by a phase difference method.Furthermore, by adding all the four detection signals, a reproducingsignal for reproducing information can be obtained.

In Embodiment 23, the polarized light hologram element also is movedsimultaneously with the movement of the objective lens. Therefore, thereis no relative movement of the dividing lines, and the amount of anoffset is small. However, even in this case, due to the influence of themovement of a light amount distribution of a semiconductor laser, anoffset occurs. In order to decrease the amount of an offset, regions inthe vicinity of the center of a light beam are replaced, wherebydetection is performed. Because of this, the influence of the movementof a light amount distribution can be reduced. In this case, byreplacing the regions at diagonally opposed positions, a FE signalaccording to the astigmatism method and a TE signal according to thephase difference method are not influenced largely.

Thus, because of the configuration of Embodiment 23, a TE signal, a FEsignal, an information reproducing signal, and a phase difference TEsignal can be obtained without an offset by using small number (four) oflight-receiving portions and small number of circuits.

Embodiment 24

In Embodiment 24, the light diffracted in Embodiment 23 is condensed inthe vicinity of a diffraction-limit. Only the difference betweenEmbodiments 23 and 24 will be described. As an optical configuration, apolarized light hologram element (splitting unit) 307 is used in placeof a polarized light hologram element 501.

FIG. 61 shows a relationship between the division of a polarized lighthologram element (splitting unit) 307 and a light beam. The polarizedlight hologram element 307 is divided into 6 regions by 4 dividing lines307 a, 307 b, 307 c, and 307 d. Among them, the regions 620 a and 620 bcorrespond to a first region mainly containing a TE signal component.The first region has no grooves of the hologram, and light beams passtherethrough completely. The regions 621 a, 621 b, 621 c, and 621 dcorrespond to a second region mainly containing an offset component of aTE signal. The second region has blazed grooves. In the second region, alight beam is diffracted in a particular direction, and is provided withastigmatism so as to previously cancel the astigmatism provided by thedetection lens 107 shown in FIG. 58. The second region is divided into 4regions by a dividing line 307 b substantially parallel to a tracktangent direction and a dividing line 307 d substantially parallel to adirection orthogonal to a track.

FIG. 62 shows configurations of the photodetector tight detector) 30 andan electric circuit. The light having passed through the region 620 a ofthe polarized light hologram element 307 becomes a light beam 640 a thatextends across light-receiving portions 30 a and 30 b. The light havingpassed through the region 620 b becomes a light beam 640 b that extendsacross light-receiving portions 30 c and 30 d. Thus, the light havingpassed through the regions 620 a and 620 b mainly containing a TE signalcomponent is split into 4 regions by the dividing lines 307 a and 307 csubstantially parallel to a track tangent direction on the hologramelement 307 and the dividing line 531 substantially parallel to adirection orthogonal to a track on the photodetector 30.

On the other hand, the light having passed through four regionssandwiched by the dividing lines 307 a and 307 c is placed at a positiondiagonally opposed to the position of the light having passed throughthe first region. The respective light beams having passed through thefour regions have their astigmatism (provided by the detection lens 107)cancelled and are condensed in the vicinity of a diffraction-limit. Morespecifically, the light having passed through the region 621 a becomes alight beam 641 d and is received by the light-receiving portion 30 c.The light having passed through the region 621 b becomes a light beam641 c and is received by the light-receiving portion 30 b. The lighthaving passed through the region 621 c becomes a light beam 641 b and isreceived by the light-receiving portion 30 c. The light having passedthrough the region 621 d becomes a light beam 641 a and is received bythe light-receiving portion 30 a.

An IV amplifier 130 receives the current signal from the light-receivingportion 30 a and outputs a voltage signal A. An IV amplifier 131receives the current signal from the light-receiving portion 30 b andoutputs a voltage signal B. An IV amplifier 133 receives the currentsignal from the light-receiving portion 30 c and outputs a voltagesignal C. An IV amplifier 132 receives the current signal from thelight-receiving portion 30 d and outputs a voltage signal D. Adders 550and 551 and a differential arithmetic operation portion 522 subjectthese signals to an arithmetic operation to obtain a signal{(A+C)−(B+D)} as a TE signal.

In Embodiment 24, a FE signal, a RF signal, and a TE signal according tothe phase difference method are obtained in the same way as inEmbodiment 23, and a push-pull signal with less offset is obtained.Furthermore, the light beams 641 a to 641 d are condensed. Therefore,even when a focusing shift and a positional shift of a photodetectoroccur, a light beam does not extend off a light-receiving portion, and aTE signal can be obtained stably.

More specifically, even when an ambient temperature is changed in thecase of using an optical recording medium with a low reflectance, anoffset fluctuation of a TE signal is small, so that an opticalinformation apparatus capable of recording/reproducing information withhigh reliability can be realized.

A division pattern for a light beam is not limited to those described inEmbodiments 20 to 24. The same effect can be obtained even with otherdivision patterns of a hologram. In particular, herein, the case hasbeen described in which a region (first region) mainly containing a TEsignal includes all the regions where a TE signal is generated, and aregion (second region) mainly containing an offset component of a TEsignal does not include a region where a TE signal is generated.However, the present invention is not limited thereto. The first regionmay include only a part of a region where a TE signal is generated, andthe second region may include a part of a region where a TE signal isgenerated.

Furthermore, it is not necessary to use all the regions in a light beamso as to generate a TE signal. For example, even in the case where thevicinity of the center of a light beam is not used for generating a TEsignal, the present invention is applicable, and its effect can beobtained.

In Embodiment 24, as a unit for splitting a light beam, a hologramelement and a prism are used. However, a light-receiving portion of aphotodetector may be divided so as to be used as a splitting unit. Inthis case, in order to change the efficiency for a light beam to reach aphotodetector, a filter having different transmittances may be used, andthe transmittance of a beam splitter may be changed partially.

Embodiment 25

FIG. 63 shows an exemplary configuration of an optical pick-up head 404constituting an optical information apparatus, as an example of stillanother optical information apparatus of the present invention.

A light source 1 emits a linearly polarized divergent beam 70 with awavelength λ of 405 nm. The divergent beam 70 emitted from the lightsource 1 is collimated by a collimator lens 53 with a focal length f1 of15 mm. The collimated beam 70 passes through a polarized beam splitter52 and a quarter-wavelength plate 54 to be converted to circularlypolarized light. Then, the beam 70 is converted to a convergent beam byan objective lens 56 with a focal length f2 of 2 mm, passes through atransparent substrate 40 a provided on an optical recording medium 40,and is condensed onto an information recording surface 40 b. The openingof the objective lens 56 is limited by an aperture 55, and a numericalaperture NA is set to be 0.85. The transparent substrate 40 a has athickness of 0.1 mm, and a refractive index n of 1.62.

The beam 70 reflected from the information recording surface 40 b passesthrough the objective lens 56 and the quarter-wavelength plate 54 to beconverted to linearly polarized light whose plane of polarization isshifted by 90° from an ingoing path, and thereafter, is reflected fromthe polarized beam splitter 52. A large part of the beam 70 reflectedfrom the polarized beam splitter 52 passes through a beam splitter 108to be a 0th order diffracted light beam 700, and a part thereof isdiffracted to generate a 1st order diffracted light beam 701. The beams700 and 701 having passed through the beam splitter 108 pass through adetection lens 59 with a focal length f3 of 30 mm and a cylindrical lens57 to be incident upon a photodetector 46. The beams 700 and 701 areprovided with astigmatism when passing through the cylindrical lens 57.

FIG. 64 schematically shows a configuration of the beam splitter 108.The beam splitter 108 has 7 divided regions 108 a to 108 g. Referencenumeral 109 denotes a beam passing through the beam splitter 108. Thebeam splitter 108 transmits a large part of the incident beam 70 togenerate a 0th order diffracted light beam 700 for generating a RFsignal, and diffracts a part thereof to generate 1st order diffractedlight beams 701 a to 701 f for generating a TE signal from the regions108 a to 108 f. In FIG. 64, h denotes a diameter of a beam passingthrough the beam splitter 108, hr denotes a length in a radius directionof the optical recording medium 40 in the region 108 g, and ht denotes alength in a track direction of the optical recording medium 40 in theregion 108 g. In Embodiment 25, the diffraction efficiencies of 0thorder diffracted light and 1st order diffracted light in the regions 108a to 108 f of the beam splitter 108 at hr/h=0.35 and ht/h=0.65 are 80%and 20%, respectively, and the diffraction efficiency of 0th orderdiffracted light in the region 108 g of the beam splitter 108 is 100%.That is, the beam 70 is allowed to pass through the region 108 g in thevicinity of the center of the beam splitter 108, whereby the diffractionefficiency of 0th order diffracted light is set to be higher than thatin the regions 108 a to 108 f on the outer circumferential side of thebeam 70.

FIG. 65 schematically shows a relationship between the photodetector 46and the beams 701 a to 701 g and 700. The photodetector 46 has 8light-receiving portions 46 a to 46 h in total. The light-receivingportions 46 a to 46 d receive the beam 700. The light-receiving portion46 e receives the beam 701 b. The light-receiving portion 46 f receivesthe beam 701 a. The light-receiving portion 46 g receives the beams 701e and 701 f. The light-receiving portion 46 h receives the beams 701 cand 701 d. The light-receiving portions 46 a to 46 h output currentsignals I46 a to I46 h in accordance with the respectively receivedlight amount.

A FE signal is obtained by the astigmatism method using the signals I46a to I46 d output from the photodetector 46, i.e., by an arithmeticoperation (I46 a+I46 c)−(146 b+I46 d).

Furthermore, a TE signal is obtained by an arithmetic operation (I46g−146 h)−k·(I46 e−I46 f). By optimizing a correction coefficient k, anoffset of a TE signal involved in the movement of the objective lens 56in a radius direction can be corrected. Furthermore, a TE signal isgenerated without using the region (the region 108 g of the beamsplitter 108) in the vicinity of the center of a beam. This is based onthe following principle: a number of fluctuation components areconcentrated in the vicinity of the center of the beam when a track tobe formed on the optical recording medium 40 is formed by beingfluctuated from a pitch tp; therefore, the fluctuation can be reduced byavoiding the use of the fluctuation component generated in the vicinityof the center of the beam. For example, in the case where a positionalshift of a track occurs every three tracks, three tracks should beconsidered as one periodic structure. The pitch in this case is threetimes of tp. The diffracted light from this periodic structure has along pitch, so that the diffraction angle of the beam is smalleraccordingly. That is, 1st order diffracted light from the periodicstructure is concentrated at the center of the beam.

A RF signal is obtained by an arithmetic operation (I46 a+I46 c+I46b+I46 d). Thus, the RF signal is generated based on the 0th orderdiffracted light 700 having passed through 7 regions 108 a to 108 g ofthe beam splitter 108.

Furthermore, the beam 70 passes through the region 108 g in the vicinityof the center of the beam splitter 108. Therefore, compared with aconventional optical information apparatus in which the beam reflectedfrom the optical recording medium 40 is split to 0th order diffractedlight and 1st order diffracted light to generate a RF signal from 0thorder diffracted light, the light amount of the 0th order diffractedlight beam 700 is increased, so that a S/N for reading informationrecorded on the optical recording medium 40 is enhanced. Accordingly, anoptical information apparatus capable of reproducing informationrecorded on the optical recording medium 40 with high reliability can berealized.

FIG. 66 shows a relationship of 3T and 8T amplitudes with respect to thediffraction efficiency of 0th order diffracted light in the region inthe vicinity of the center of a beam. The conditions are follows: 8-14modulation, 3T mark length=0.23 μm, ht/h=0.65, hr/h=0.35, and the 0thorder diffraction efficiency in the regions 108 a to 108 f other thanthe region in the vicinity of the center of a beam splitter is 80%. InFIG. 66, ● represents a standardized 3T amplitude, and ◯ represents astandardized 8T amplitude. The 3T and 8T amplitudes are standardizedwith the amplitude in the case where the 0th order diffractionefficiency of the region 108 g in the vicinity of the center of the beamsplitter 108 is 80%. By increasing the diffraction efficiency of 0thorder diffracted light in the region 108 g in the vicinity of the centerof a beam from 80% to 100%, the 3T and 8T amplitudes are improved byabout 7% and 8%, respectively. When the 0th order diffraction efficiencyin the region in the vicinity of the center of a beam is increased as inEmbodiment 25, the S/N of a RF signal can be enhanced withoutinfluencing the characteristics of a TE signal.

Furthermore, the effect in Embodiment 25 can be obtained by using anymodulation system, instead of the 8-14 modulation. In the case where themodulation system in which a shortest mark length is 2T, modulation suchas 1-7 modulation is used together with a signal detection method by apartial response (PRML), and if the condition (e.g., 2T=0.15 μm, 3T=0.25μm) under which the amplitude of a 3T signal is enhanced is set, anerror ratio is decreased noticeably.

The beam splitter 108 in Embodiment 25 may be a non-polarized typeelement, so that it can be produced by very inexpensive resin molding.Therefore, an optical information apparatus that is inexpensiveaccordingly can be provided.

In Embodiment 25, the case has been described in which the shape of theregion in the vicinity of the center of the beam splitter 108 isrectangular. However, the division pattern of the region in the vicinityof the center of the beam splitter 108 is not limited thereto. Forexample, even with the division pattern as shown in FIG. 28, the sameeffect can be obtained.

In Embodiment 25, the beam splitter 108 is placed in the optical pathfrom the polarized beam splitter 52 to the photodetector 46. However,the beam splitter 108, the quarter-wavelength plate 54, and theobjective lens 56 may be integrated with each other. In this case, thebeam splitter 108 is composed of a polarization-dependent element,whereby the beam splitter 108 is allowed to transmit all the incidentbeams 70 in an ingoing path from the light source 1 to the opticalrecording medium 40. On the other hand, in a return path in which a beamreflected from the optical recording medium 40 travels to thephotodetector 46, a large part of the beam 70 incident upon the beamsplitter 108 passes through the beam splitter 108 to be the 0th orderdiffracted light beam 700, and a part thereof is diffracted to generatea plurality of 1st diffracted light beams 701. In the case where thebeam splitter 108, the quarter-wavelength plate 54, and the objectivelens 56 are integrated with each other, the positional relationshipbetween the beam 70 and the region 108 g is kept constant at all times.Therefore, the increase ratio of a 3T signal becomes constant; andinformation recorded on an optical recording medium can be reproducedmore stably.

Embodiment 26

FIG. 67 schematically shows a beam splitter 69 constituting an opticalpick-up head, as an example of another optical information apparatus ofthe present invention. By using the beam splitter 69 in place of thebeam splitter 68 in Embodiment 16, an optical pick-up head in Embodiment26 can be configured.

Regions 69 a to 69 g of the beam splitter 69 correspond to the regions68 a to 68 g of the beam splitter 68, and generate 1st order diffractedlight. The region 69 h of the beam splitter 69 corresponds to theregions 68 h and 681 of the beam splitter 68, and generates 1st orderdiffracted light. The region 69 i of the beam splitter 69 corresponds tothe regions 68 j and 68 k of the beam splitter 68, and generates 1storder diffracted light. The beam splitter 69 is different from the beamsplitter 68 in that the width of the region 69 h of the beam splitter 69corresponding to the regions 68 h and 681 of the beam splitter 68 isenlarged, and the width of the region 69 a is smaller than that of theregion 68 a accordingly. This also applies to the relationship among theregions 68 j, 68 k, 69 i, 69 b, and 68 b.

In the case where information is recorded as a mark string in which acomplex reflectance is changed every other track as shown in FIG. 33, amark string recorded on the optical recording medium behaves as adiffraction grating in which a pitch is twice tp. Thus, a beam radiatedto the optical recording medium is diffracted by a groove-shaped trackwith a pitch of tp and a diffraction grating with a pitch of 2·tp.

In FIG. 67, broken-line regions 70 e and 70 f represent an image of 1storder diffracted light diffracted by the groove-shaped track having apitch of tp of the optical recording medium in the beam 70. On the otherhand, broken lines 70 g and 70 h represent the position of the innermostcircumference of the image of 1st order diffracted light diffracted bythe diffraction grating with a pitch of 2·tp in the beam 70. Althoughnot shown in detail for simplicity, the 1st order diffracted lightdiffracted by the diffraction grating with a pitch of 2 tp is incidentfrom the broken lines 70 g and 70 h to the outside of the beam 70 (i.e.,the region facing the regions 70 e and 70 f).

The fluctuation in symmetry of a TE signal is caused by the diffractedlight generated by the diffraction grating with a pitch of 2·tp. In theregions 69 a, 69 c, and 69 d, the amount of incident 1st orderdiffracted light diffracted by the diffraction grating with a pitch of2·tp is compared, and the incident amount in the region 69 a is muchlarger than the sum of the incident amounts in the regions 69 c and 69d. This also applies to the regions 69 b, 69 e, and 69 f.

As described above, when a TE signal is generated, the signal obtainedfrom the region 69 a and the signal obtained from the regions 69 c, 69 dare multiplied by a coefficient and subjected to subtraction. Therefore,a component to be fluctuated in a TE signal contained in the region 69 ais reduced. However, the amount of 1st order diffracted light diffractedby the diffraction grating with a pitch of 2·tp contained in the regions69 c and 69 d may be insufficient for eliminating a fluctuationcomponent from a TE signal by subtraction.

In Embodiment 26, the area of the region 69 h is enlarged. The polarityof the signal obtained from the region 69 h is the same as that of thesignal obtained from the regions 69 c and 69 d. Therefore, by enlargingthe area of the region 69 h, the fluctuation in a TE signal can bereduced sufficiently. Herein, assuming that the radius of the beam 70 onthe beam splitter 69 is 1, the width h1 of the region 70 h is set to be0.70, and the width L2 from the center of the beam splitter 69 to theinnermost side of the region 69 a is 0.40. These widths are shown merelyfor illustrative purpose, and they may be designed appropriately,considering the characteristics of an optical recording medium, thenumerical aperture of an optical system, the wavelength of a lightsource, and the like.

Even in the case where the amplitude of a TE signal is fluctuated whenthe reflectance of a mark formed on an optical recording medium ischanged, by using the beam splitter 69 in Embodiment 26, the fluctuationin a TE signal is alleviated, and an optical information apparatuscapable of performing a tracking operation stably can be provided. Theoptical information apparatus of the present embodiment is particularlyeffective in the case where an optical recording medium having a largereflectance ratio is used (i.e., the ratio of a reflectance between arecorded state and an unrecorded state is 3 or more).

It should be appreciated that, as described in Embodiment 10, theamplitude of a TE signal can be stabilized further by controlling theamplitude of a TE signal, using a variable gain amplifier.

Furthermore, herein, for simplicity of description, the case whereinformation is recorded on every other track as shown in FIG. 33 hasbeen described. However, various pitches may be used depending upon thepositional relationship between a recorded track and an unrecordedtrack. In any case, a pitch is longer than the pitch tp in terms ofequivalence, and the effect in the case of using the beam splitter 69 inEmbodiment 26 can be obtained. In particular, there is no limit torecording of information in a track. Furthermore, even in the case wherethe pitch of an unrecorded track is partially varied, the effect in thecase of using the beam splitter in Embodiment 26 can be obtained.

Embodiment 27

FIG. 68 schematically shows an example of a relationship between aphotodetector 45 constituting an optical pick-up head and the beams 75 ato 75 h, and 76 a to 76 h, as an example of another optical informationapparatus of the present invention. By using the photodetector 45 inplace of the photodetector 46 in Embodiment 11, and using a signalprocessing portion (described later) for generating a TE signal, anoptical pick-up head in Embodiment 27 can be configured.

In the optical pick-up head in the present embodiment, the beams 75 dand 75 e are received by one light-receiving portion 45 g in the sameway as in the optical pick-up head in Embodiment 11, and the beams 75 fand 75 g are received by one light-receiving portion 45 h, respectively.Abeam used for generating a TE signal is not incident upon thelight-receiving portions 45 i and 45 j.

FIG. 69 shows a configuration of a signal processing portion forgenerating a TE signal. This signal processing portion is different fromthe signal processing portion in Embodiment 10 described with referenceto FIG. 23 as follows: the beams 75 d and 75 e are received by onelight-receiving portion 45 g, and the beams 75 f and 75 g are receivedby one light-receiving portion 45 h, so that the adders 802 and 803 arenot necessary; and differential arithmetic operation portions 814 to 817and variable gain amplifiers 818 to 821 are provided.

The differential arithmetic operation portions 814 to 817 receivesignals output from the light-receiving portions 45 e to 45 h andsubtract the signal output from the light-receiving portion 45 i fromthe signals output from the light-receiving portions 45 e to 45 h.Unnecessary stray light, such as light randomly reflected from aperipheral portion of an optical component constituting the opticalpick-up head, natural light emitted from a light source, and the like,may be incident upon the light-receiving portions 45 e to 45 i. Thisunnecessary stray light is mostly composed of a largely diverged beam onthe photodetector 45, and substantially the same amount of stray lightis incident upon the light-receiving portions 45 e to 45 i. Thelight-receiving portion 45 i does not receive a beam used for generatinga TE signal. Therefore, the signal output from the light-receivingportion 45 i is caused by stray light. By providing the differentialarithmetic operation portions 814 to 817, a signal caused by stray lightcan be reduced from the signal output from the light-receiving portions45 e to 45 h. Since the light-receiving portion 45 j is not used, it isnot necessarily required to provide the photodetector 45.

The signals output from the differential arithmetic operation portions814 to 817 are input to the variable gain amplifiers 818 to 821, andadjusted to desired signal levels. Herein, the gains of the variablegain amplifiers 820 and 821 are adjusted so that the signal levels basedon the light amounts of the beams divided by a beam splitter incidentupon the light-receiving portions 45 e and 45 f are equal to each other,when actuators are placed in a neutral position. Similarly, the gains ofthe variable gain amplifiers 818 and 819 are adjusted so that the signallevels based on the light amounts of the beams divided by the beamsplitter incident upon the light-receiving portions 45 g and 45 h areequal to each other. The methods for processing the signals output fromthe variable gain amplifiers 818 to 821 are the same as those inEmbodiment 10. Even in the case where defects and fingerprints, whichmay change a reflectance, adhere to an optical recording medium,unintended fluctuation is caused less in the signals output from thedifferential amplifiers 804 and 801, and a stable tracking operation canbe performed.

Herein, the light-receiving portion 45 i is used for detecting straylight; however, the light-receiving portion 45 j may be used in place ofthe light-receiving portion 45 i.

Furthermore, since the sizes of the light-receiving portions 45 e to 45h, and 45 i are set to be the same, a differential arithmetic operationis performed simply. However, the same effect can be obtained even inthe case where both the light-receiving portions 45 i and 45 j are used(i.e., a light-receiving portion with a double area is used) fordetecting stray light, and the signals output from the light-receivingportions 45 i and 45 j are subjected to a differential arithmeticoperation after being attenuated to ½. By enlarging the area of alight-receiving portion for detecting stray light, the influence of thebias in the distribution of stray light can be alleviated. Therefore, asignal caused by stray light can be eliminated with good precision.

Embodiments 1 to 27 described above are examples, and variousembodiments can be used in a range without departing the spirit of thepresent invention. It should be appreciated that various alterationssuch as the use of a non-polarization optical system may be made in arange without departing the spirit of the present invention. A FE signaldetection system other than the astigmatism method has not bee describedsince it is not related to the spirit of the present invention. However,there is no particular limit to a detection system of a FE signal. Allthe general FE signal detection systems, such as a spot size dictationmethod, a Fourcault method, and the like, can be used.

Furthermore, even in the case of using an optical recording medium inwhich a TE signal amplitude is fluctuated when there is a variation in aposition, a width, and a depth of a track in the course of production ofthe optical recording medium or information is recorded in a track, thefluctuation in a TE signal amplitude can be reduced, and a stabletracking operation can be performed in all the optical informationapparatuses of the above-mentioned embodiments. Therefore, aninexpensive optical recording medium can be provided in improved yield.

Furthermore, an optical recording medium in which a TE signal amplitudeis fluctuated can be used according to the present invention. Therefore,an original master of an optical recording medium can be cut with alaser beam at a high speed, instead of cutting with an electron beam.This enables an original master to be produced at a low cost, and anoptical recording medium that is inexpensive accordingly can beprovided.

Herein, the wavelength λ of the light source 1 is set to be 405 nm, andthe numerical aperture NA of the objective lens 56 is set to be 0.85.The optical information apparatus of the present embodiment exhibits thefeatures described above remarkably, in particular whentp/0.8<λ<λ/NA<0.5 μm.

Furthermore, when a diffraction element is used for a beam splitter,±1st order diffracted light generally is generated. However, in the caseof using conjugate light, light-receiving portions receiving therespective beams of conjugate light may be provided. In the case whereconjugate light is not used, a diffraction element is blazed so as toenhance a light use efficiency.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. An optical information apparatus, comprising: an optical pick-up headincluding: a light source emitting a light beam; a condensing unitreceiving beams from the light source and condensing the beams onto anoptical recording medium; a beam splitter receiving the beams reflectedfrom the optical recording medium and splitting the beams; and aphotodetector receiving the beams split by the beam splitter in aplurality of light-receiving portions and outputting signals inaccordance with amounts of the received beams; and a tracking errorsignal generator generating a tracking error signal for irradiating adesired track with a beam, the optical recording medium having tracks ata track pitch of tp, wherein assuming a signal proportional to adifference between light amounts of two regions divided by a dividingline that passes through a center of the beam and is parallel to thetrack is a push-pull signal, and an amplitude of the push-pull signal isfluctuated when the light beam is scanned in a direction orthogonal tothe track of the optical recording medium, the tracking error signalgenerator performs a differential arithmetic operation with respect tosignals output from the light-receiving portions, without using a regionin a vicinity of the center of the beam, thereby generating a trackingerror signal.
 2. An optical information apparatus, comprising: anoptical pick-up head including: a light source emitting a light beam; acondensing unit receiving beams from the light source and condensing thebeams onto an optical recording medium; a beam splitter receiving thebeams reflected from the optical recording medium and splitting thebeams; and a photodetector receiving the beams split by the beamsplitter in at least four light-receiving portions and outputtingsignals in accordance with amounts of the received beams; and a trackingerror signal generator generating a tracking error signal forirradiating a desired track with a beam, the optical recording mediumincluding an information recording surface for recording information,and a reflective surface for reflecting the beams when the beams arecondensed on the information recording surface, in addition to theinformation recording surface on which the beams are condensed, theoptical recording medium having tracks at a track pitch of tp, whereinassuming that a signal proportional to a difference between lightamounts of two regions divided by a dividing line that passes through acenter of the beam and is parallel to the track is a push-pull signal,and an amplitude of the push-pull signal is fluctuated when the lightbeam is scanned in a direction orthogonal to the track of the opticalrecording medium, the tracking error signal generator obtains a trackingerror signal by an arithmetic operation: (I1−I2)−k·(I3−I4), where I1 toI4 are outputs of the four light-receiving portions receiving the beamsand k is a real number, without using a region in a vicinity of thecenter of the beam for the signals of I1 to I4, and the fourlight-receiving portions do not receive the beams reflected by thereflective surface for reflecting the beams when. the beams arecondensed on the information recording surface.
 3. An opticalinformation apparatus, comprising: an optical pick-up head including: alight source emitting a light beam; a condensing unit receiving beamsfrom the light source and condensing the beams onto an optical recordingmedium; a beam splitter receiving the beams reflected from the opticalrecording medium and splitting the beams; and a photodetector receivingthe beams split by the beam splitter in at least four light-receivingportions and outputting signals in accordance with amounts of thereceived beams; and a tracking error signal generator generating atracking error signal for irradiating a desired track with a beam, theoptical recording medium including an information recording surface forrecording information, and a reflective surface for reflecting the beamswhen the beams are condensed on the information recording surface, inaddition to the information recording surface on which the beams arecondensed, the optical recording medium having tacks at a track pitch oftp, an opening limit member is provided between the condensing unit andthe photodetector, wherein assuming that a signal proportional to adifference between light amounts of two regions divided by a dividingline that passes through a center of the beam and is parallel to thetrack is a push-pull signal, and an amplitude of the push-pull signal isfluctuated when the light beam is scanned in a direction orthogonal tothe track of the optical recording medium, the tracking error signalgenerator obtains a tracking error signal by an arithmetic operation:(I1−I2)−k·(I3−I4), where I1 to I4 are outputs of the fourlight-receiving portions receiving the beams and k is a real number,without using a region in a vicinity of the center of the beam for thesignals of I1 to I4, and the four light-receiving portions do notreceive the beams reflected by the reflective surface for reflecting thebeams when the beams are condensed on the information recording surface.4. An optical information apparatus, comprising: an optical pick-up beadincluding: a light source emitting a light beam; a condensing unitreceiving beams from the light source and condensing the beams onto anoptical recording medium; a beam splitter having a plurality of regions,receiving the beams reflected from the optical recording medium andsplitting the beams; and a photodetector receiving the beams split bythe beam splitter in at least four light-receiving portions andoutputting signals in accordance with amounts of the received beams; anda tracking error signal generator generating a tracking error signal forirradiating a desired track with a beam, the optical recording mediumincluding an information recording surface for recording information,and a reflective surface for reflecting the beams when the beams arecondensed on the information recording surface, in addition to theinformation recording surface on which the beams are condensed, theoptical recording medium having tracks at a track pitch of tp, whereinassuming that a signal proportional to a difference between lightamounts of two regions divided by a dividing line that passes through acenter of the beam and is parallel to the track is a push-pull signal,and an amplitude of the push-pull signal is fluctuated when the lightbeam is scanned in a direction orthogonal to the track of the opticalrecording medium, the beam splitter splits the beam in a directiondifferent from that of the light-receiving portion outputting a signalfor generating the tracking error signal over a width h of a region in avicinity of a center to be irradiated with the beam, a width S of thefour light-receiving portions outputting a signal for generating thetracking error signal has a relationship:S≦2·h·α·NA·d/(D·n2)  (1) where D is a size of the beam on the beamsplitter; NA is a numerical aperture of the condensing unit; α is alateral multiplication of an optical system in the optical pick-up headfrom the optical recording medium to the photodetector; d is an intervalbetween the information recording surface and the reflective surface;and n2 is a refractive index present in the interval d between theinformation recording surface and the reflective surface, and thetracking error signal generator obtains a tracking error signal by anarithmetic operation: ((I1−I2)−k·(I3−I4), where I1 to I4 are outputs ofthe four light-receiving portions receiving the beams and k is a realnumber, without using a region in a vicinity of the center of the beamfor the signals of I1 to I4.
 5. An optical information apparatus,comprising: an optical pick-up head including: a light source emitting alight beam; a condensing unit receiving beams from the light source andcondensing the beams onto an optical recording medium; a beam splitterhaving a plurality of regions, receiving the beams reflected from theoptical recording medium and splitting the beams; and a photodetectorreceiving the beams split by the beam splitter in at least fourlight-receiving portions and outputting signals in accordance withamounts of the received beams; and a tracking error signal generatorgenerating a tracking error signal for irradiating a desired track witha beam, the optical recording medium including an information recordingsurface for recording information, and a reflective surface forreflecting the beams when the beams are condensed on the informationrecording surface, in addition to the information recording surface onwhich the beams are condensed, the optical recording medium havingtracks at a track pitch of tp, an opening limit member being providedbetween the condensing unit and the photodetector, wherein assuming thata signal proportional to a difference between light amounts of tworegions divided by a dividing line that passes through a center of thebeam and is parallel to the track is a push-pull signal, and anamplitude of the push-pull signal is fluctuated when the light beam isscanned in a direction orthogonal to the track of the optical recordingmedium, the beam splitter splits the beam in a direction different fromthat of the light-receiving portion outputting a signal for generatingthe tracking error signal over a width h of a region in a vicinity of acenter to be irradiated with the beam, a width S of the fourlight-receiving portions outputting a signal for generating the trackingerror signal has a relationship:S≦2·h·α·NA·d/(D·n2)  (1) where D is a size of the beam on the beamsplitter; NA is a numerical aperture of the condensing unit; α is alateral multiplication of an optical system in the optical pick-up headfrom the optical recording medium to the photodetector; d is an intervalbetween the information recording surface and the reflective surface;and n2 is a refractive index present in the interval d between theinformation recording surface and the reflective surface, and thetracking error signal generator obtains a tracking error signal by anarithmetic operation: (I1−I2)−k·(I3−I4), where I1 to I4 are outputs ofthe four light-receiving portions receiving the beams and k is a realnumber, without using a region in a vicinity of the center of the beamfor the signals of I1 to I4.